The specific computer I am using is a Dell 8400. It does have SATA drives, but it is probably 5 years old and probably runs the 1st generation SATA stuff. So a newer computer probably has 2nd generation SATA hardware. No special speed drives or anything like that.
I run XP Pro and 1 GB ram (although I did recently buy another gig to throw in there - no good reason other than it is cheap now - I still need to install it however). I have 1 HDHomeRun (running as 2 QAM tuners) and a HD-PVR. I routinely record 2 HD shows at the same time and about once per week it records 3 HD shows at once. Without the use of an extender to watch these recordings, my computer would not be able to support all that I do. But since the extender takes most of the major processing load off the computer, it can be a much slower machine. Also, having the HDHomeRuns help too because they are network recorders. All the computer is doing is physically saving the 1's and 0's to the harddrive.