I received the Asus 5550 card over the weekend and got it installed. So far, it's worked flawlessly - nice, smooth video, Dolby Digital audio from ripped DVD's, no incompatibilities so far, etc.
The only problem is that the system is a bit sluggish running MC7; it's a Dell Optiplex GX280 SFF with a P4HT, 4 GB of RAM, and a 2TB hard drive. In everything other than MC, it's pretty quick. I went ahead and got an upgrade system on eBay - an Optiplex 755 with a Core 2 Quad 9400; I'll end up with 4 or 6 GB on that system with Win 7 64 Enterprise, the Asus video card, and the 2TB drive from the current system. The system also has a MC remote control and a wireless keyboard with trackball. It's all connected via HDMI to a Pioneer Elite VSX-23TXH, an Epson 705 HD projector, and a 19" LCD TV mounted in the in-wall shelf unit (thanks to the dual HDMI outputs on the Pioneer). I got a good deal on the TV from Newegg... to good a deal to pass up to keep from having to turn on the projector to play music, set the DVR, etc.
I started upgrading all this earlier this year. I started out with a tired old Zenith SD RPTV, a Sherwood DPL receiver from '95, and a standard old DVD player with no PC integration at all. It's turned out to be a very nice system that never fails to impress with audio and video (as well as the 'cool' factor). Much of this has been possible from what I've learned on this site... thanks guys!