Wow, good timing. I've been testing a variety of cards on Vista X64 since before beta shipped, and now I'm testing them on Win7 as well. I have most of the cards on this thread.
E-Mu 0404 PCI - best sound quality (unmodded) that I have had in a PCI card when it works. Bass is well controlled, nice clear treble. It's a little better than the Audiophile 2496, and noticably cleaner than the Revolution 7.1.
Vista X64 - directsound streams occasionally get corrupted. The worse offender for me is Left 4 Dead. Usually within 10-30 minutes, it will output garbage for a few seconds, sometimes up to a few minutes. There's a dumb bug that causes the required PatchMixDSP application to crash if you connect through remote desktop or Windows Home Server. I reported it in 2007 through their driver beta forums, and it's still not fixed.
Win7 beta - Vista drivers install, but the audio corruption is much more frequent and happens every time I try to play an MP3 with Windows Media Player. Same patchmix issue.
M-Audio Audiophile 2496 - Not bad sound quality, and a steal since you can get them used for well under $100. However - it took M-Audio from 2006 to 2008 to release a driver for this, and its not stable. I haven't been able to A-B it with the E-mu 0404 since I haven't had it long, and ran into the issues below.
Vista (x86/x64) - avoid. There are bugs in the driver that cause bluescreens when the system enters sleep mode. Multiple reports on the forums and M-Audio has stated that they "don't support sleep mode"
Delta 192 Audiophile Vista SP1 resume failure - The Official M-Audio User Forums
Win7 beta - same problems continue
M-Audio Revolution 7.1 - This was my first good sound card. Loved it on XP, but the E-mu 0404 PCI was slightly better and gave me better I/O. Drivers for Vista x64 are available in beta on M-Audio forums.
Vista - avoid. The same sleep issue with the M-Audio Audiophile 2496 affects all of their Envy24-based boards (Delta 44, 66, 1010, 410, 1010lt, Audiophile 2496, Audiophile 192, Revolution 5.1, Revolution 7.1)
Chaintech AV710 - Better than onboard. The rear channel was almost as good as the revolution 7.1's quality. Drivers don't support ASIO by default, so you will need to install ASIO4ALL. J.River Media Center, the Chaintech AV710, and ASIO4ALL was a good combo on XP.
Vista - the last time I checked (2008), drivers did not support all card features. It only worked at 16/48, and had issues with digital output. Not sure if it was ever fixed. Unfortunately, I gave this card away to an XP media center user, so I can't retest.
Asus Xonar D1 - This is en-route between CA and WA right now. My hope is that it's rock stable, and based on reviews, it's supposed to be pretty solid. We'll see how it compares to the above.