Over here, we've got Samplitude running on AMD Barton 2500+ and Athlon 64 3400+ systems (ASUS mobos) - very fast, and more processing power than we need. Also, have never had a crash of any sort. I haven't tried a test of maximum simultaneous record tracks nor of maximum plugins, but we've had sessions with a number of room-simulator reverbs, eqs and compressors on all 32 tracks (24 bit 96k), and a whole host of Mix Buss and Aux effects without pushing the limits of the CPU load.
Protools 6.1 LE also works great on these systems, as does Ableton Live.
I've heard that the same systems, though, when used extensively for email and web surfing and graphics stuff in addition to audio, start to develop unique "personalities" that aren't desirable in a professional audio recording environment... so make it lean and you'll be much happier. The same probably goes for Intel-based computers, though, too (and probably has a lot more to do with hard drive fragmentation and XP resources than with the chipsets)