I use a lowly dell 2500 with the midiman quattro usb. I only do stereo output, but now that I have jack working via the ccrma capabilities kernel I may be able to process live input at the same time as well. The quattro isn't for doing heavy multi-tracking, the hdsp with the pc card is a better (but more expensive) choice for doing that. With the capabilities (not the low latency) ccrma kernel the quattro works great with both input and output, and jack works well, though it may perform better when I can upgrade to the maximum 500 megs of ram for my old machine. You don't need the absolute optimal, or most expensive new system, you can get by on a used laptop, or build a pc from scratch, I built my desktop system for $80 from surplus parts, but its still got a 1.1 gigahtz athalon, so you can build a decent (but not the latest) machine for almost pennies compared to what it used to cost to get a machine. You can get barbones shuttle systems for cheap, with the handle for carrying the small box to a gig, that with a flatscreen monitor is cheaper and more robust than a laptop.
