PC's for live performance -- I've tried it a few times already. Although nothing really went wrong, I'm still not sure if I wanna use my laptop (T7400 w/ 2gb RAM) in a gig on a regular basis. Too many things can go wrong -- memory or hard drive failure, software or hardware bugs, etc.. I'd dread to see that happen on a crucial part of the show. I've stopped acquiring hardware synths ever since I got my xv3080... but lately I've been getting new stuff (some of them 2nd hand) to upgrade my live stuff -- a Motif Rack, a Triton Extreme and a Fantom XR. Then I sold the XV.
If you really need to use a PC, I'd say for the time being, try not to use disk streaming. At least you're eliminating one variable, one thing that might fail in the middle of a gig. Use reason, or if you have to stream from the hard disk, use a large buffer or preload size, to minimize as much streaming as you can. In other words, try to get your RAM to at least 2GB, so you can load a lot of samples.
As for me, i'll load my samples either on the Triton Extreme (96MB) or the Fantom XR (528MB). When I need more, I'll pass on the gig to somebody who has more... haha.