A very flexible solution is a USB-DAC as almost any PC offers USB.
Using a USB DAC you don't need a sound card at all.
If you use a laptop this is probably the way to go if you want to improve on the onboard sound card.
Most USB-DAC's accept 16 bit/32kHz, 44.1 kHz, 48 kHz
Some accept 24 bit/96 kHz
Very few support sample rates bigger than 96 kHz. Simply because in this case you have to write your own USB audio driver for the PC as all native mode USB audio drivers are limited to 96 kHz. The new USB audio 2 standard (spring 2009) supports 176/192 but as far as I know it is not implemented in Linux, OSX or Win yet.
Redbook audio (CD) is 16 bits/44.1 kHz.
Makes you wonder why you need a 24 bit DAC.
However, more and more websites offers recording in the original bit depth and this is 24 bits most of the time.
Recordings are in general made with a higher sample rate than 44.1, 88.2 and 96 kHz are very common.
24 bit/192 kHz upsampling
Lot’s of people think this spec applies to the USB input too only to find out they get no sound at all when playing high res recording in their native format.
24 bit/192 kHz upsampling is about the way the input is treated internally.
This has nothing to do with the accepted input over the USB.
If there is no explicit information about the capabilities of the USB input, you better assume it is limited to 16/48.
Will playing 16 bits sources on a 24 DAC affect sound quality?
Very unlikely, all what is done is padding 8 zero’s too the 16 bits to make it a 24 bit word.
In fact there might even be a benefit, if you apply digital volume control, you can chop off these eight zero’s (48 dB reduction) without loosing resolution.
More on USB audio can be found here.
Configuring USB audio can he found here.
A clear and well written step by step guide to setup and USB DAC using XP, Vista or OSX can be found at the Ayre website.

Ayre QB-9 $2500Digital input: Asynchronous USB up to 24/96
A upgrade to 24/192 will be available end 2009. This require an operating system supporting Class 2.0 USB audio.
Analog output: RCA (1 pair), balanced XLR (1 pair).
Digital input:
Analog output: balanced (XLR) and unbalanced (RCA)
A discussion between forum members and Elias Gwinn of Benchmark Media systems can be found here.

Digital input:
USB2.0 interface on a B-type connector. Operates in asynchronous mode.
AES3 on a 3-pin female XLR connector.
4x SPDIF on 2x RCA Phono, 1x BNC connectors and 1x TosLink optical connector.
All digital inputs will accept PCM data at up to 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.
Data from any input may be converted to 24 bit PCM at 32, 44.1, 48, 88.2, 96, 176.4 or 192kS/s or DSD (1 bit data at 2.822MS/s). The output sample rate must be equal to or greater than the input sample rate.
Digital output:
IEEE 1394 interface on 2x 6-way connectors. In DSD mode, the interface outputs dCS-encrypted DSD (1 bit data at 2.822MS/s).
2x AES3 on 3-pin female XLR connectors. Each outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s,
OR as a Dual AES pair at 88.2, 96, 176.4 or 192kS/s.
2x SPDIF on RCA Phono and BNC connectors. Each outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.
1x SDIF-2 interface on 2x BNC connectors, outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.
It is a upsampler so you still need a DAC for the conversion to analogue. Well, they have one:
Scarlatti D/A-Converter € 18.900,-
At € 29.800,- you have a USB DAC.
If you decide to synchronize the two with a Scarlatti Master Clock: add € 8.500.

USB: 24 bits 96 kHz
Full support for both adaptive and asynchronous transfer modes.
Balanced output (Tini Q)
One of the very few DAC's accepting 24/192 over USB. You need to install the custom USB driver to do so.
The Monitor 01 USD comes without the headphone out.
One of the few USB DAC's supporting 24/192.
Available on eBay.
Some comment by Thorsten
2 XLR mic inputs with phantom power
All models are using asynchronous USB.
Brick V2 $1750,-
16/44.1 NOS (Non oversampling) DAC (TDA1543N2)
12AU7A/ECC82 output tube
Analog output: RCA
The Crimson and the Cosecant are available with the same TDA1543N2 or a Wolfson DAC.
In the latter case the USB support 24 bits/96 kHz
The Crimson is planned to have a ESS Sabre DAC.
Now, for the first time ever, Wavelength Audio has developed Asynchronous Mode USB Audio. This means the computer is controlled by the USB DAC. No longer is the tail wagging the dog. Instead, an ultra-low-jitter audio master clock located in the DAC controls the audio transfer rate from the computer. Jitter is reduced by a factor of greater than 100 times! What's more, this is accomplished using the standard USB drivers (Windows or MacIntosh) for easy plug-and-play installation. Now the convenience of computer-based audio is combined with the lowest possible jitter.

Source: Wavelength
DAC are listed in this section because the manufacturer states that it is a 16 bit DAC or the manufacturer is completely unclear about bit depth and sample rate supported so it probably is 16 bits/ 48kHz.
Texas Instruments PCM1744
USB: 16 bits/48 kHz
MusicStreamer+ $249.95
TI DAC PCM1794
USB: 16 bits/48 kHz
192 kHz Upsampling DAC / Headphone Amp with USB and S/PDIF inputs.
Beresford
TC-7520 £149.99
Inputs: USB/COAX/TOSLINK SPDIF: 24bit/96kHz
USB: 16/48
Seperate re-clocking circuits for USB and SPDIF
Output: line, class A head- and pre-amp.
USB 16 bits/ 32, 44.1, 48 kHz PCM
DAC: Burr Brown PCM2704.
Output: headphone
out
Digital out: Optical, Coax (RCA and BNC) and XLR.

USB: 16-bit / 32, 44.1, 48 kHz
DAC: non-oversampling (NOS). Audio playback occurs at the recorded frequency (), with no resampling, upsampling, or oversampling.
Output: RCA
32 bit Texas Instruments Digital Signal Processor (DSP)
DAC: Twin Wolfson WM8740
Digital input:
2x S/PDIF (coax & Toslink)USB input connects direct to PC without drivers
| Digital input word widths supported: | 16 -24bit (16 bit for USB) |
| Digital input sampling frequencies supported: | 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz. |
| USB input | 16 bits / 44.1,48kHz |
| Audio output up-sampling: | Fixed 24bit/192kHz |
Output
Analog: RCA/XLR
Digital: 1x S/PDIF (coax & /Toslink)
Digital input
Sampling rate: 44kHz – 96kHz
Analog output: RCA
Digital input
DAC
Analog output:
Class A discrete FET based output stage
Balanced (XLR) and unbalanced (RCA)
Upsampling: 24 bit/192 kHz
Fully balanced design
Digital input:
Dual Mono DAC (2 x Burr Brown PCM1796)
USB-Codec: Texas-Instruments PCM2906 Transceiver
Analog output:
Single ended Class A amplifiers
balanced (XLR) and unbalanced (RCA)
Digital output: COAX, TOS, AES/EBU-Input are routed to the USB-output (up to 48kHz/16bit).
Digital input: USB, Optical, Coaxial, AES/EBU(XLR)
Analog output: balanced (XLR) and unbalanced (RCA)
32, 44.1, 48, 88.2, 96, 176, 192K sampling
Digital input:
USB (1), COAX (2), OPTICAL (2),AES-EBU (1) BNC (2)
Sample rates: 16-24Bit PCM, 16Bit 32K-48K USB
Analog output: balanced (XLR) and unbalanced (RCA)

Digital input (5): AES/EBU (XLR), S/PDIF (BNC, RCA, TosLink), USB.
Sample rates: 32–96kHz (S/PDIF, AES/EBU), 32kHz–48kHz (USB).
Analog output: RCA (1 pair), balanced XLR (1 pair).
Digital input: Asynchronous USB
No oversampling and no analog or digital filter
16bit @ 44.1kHz WAV/ FLAC only
Analog output: RCA (1 pair)
Digital input:
2 x SP/DIF BNC Coax
2 x AES Balanced XLR Input
2 x Plastic fibre optic ( TOSlink )
1 x USB ( B type )
1 x Bluetooth supporting A2DP Stereo Audio
Sample frequencies 32kHz – 96kHz Single Cable,
176kHz & 192kHz Dual Cable ( Dual data mode )
Random master clock jitter of less than 3pS cycle to cycle
Analog output: 2 X RCA /
2 X balanced (XLR)