I've worked with Avid Xpress, but it was on a 32-bit platform. Actually, it was a Mac G4 to be exact. But i prefer the PC format. The Avid will run faster on the AMD 64 if the code is written for 64-bit, that's for sure. In fact, benchmarks have shown significant improvement on video editing features on Windows 64-bit beta, and that's with 32-bit programs. If there is a 64-bit Avid, then that would fly on the AMD 64. Personally, I'd recommend a dual AMD Opteron solution for Avid, perhaps a dual opty 244. I'd also increase the RAM to at least 1 GB, and it doesn't need to have fast timings, just good bandwidth. And if you're going to cut a feature in HD, then 200 GB might be cutting it close. 200 GB was good for my project which was 2 hours in length and at 720 X 480. Since HD is at 1080 X whatever, then you'll probably need a little more room.
3Dmark 2001: 30015