you need to go 64 bit and add ram. cs5 will work much better.
I went from win7 + 4 gb ddr2 ram + 512 mb nvidia 9600 gt card PC rig to mbpro (8gb ddr3 ram, 512 mb ati card, 2,0 ghz quadcore cpu) -
start-up times sped up quite a lot (approx 10 sec first run, less than 5 on 2nd - and I mean quitting photoshop, not just closing documents)
brushes still have some lag, and mixer brushes lag too - but at least I can go up to 80 px, not 30-40 without getting slowdowns.
windows versions also had some sort of quickly building up memory leak, that was eating my 3 + gigs, allocated for PS, quite quickly.
upgrading might be expensive, but a general rule with every CS update since cs3 is investing into both software and hardware, otherwise your tech might be too outdated for adobe's latest programs. (aftereffects cs5 doesn't even install on 32-bit systems, for example.)
on the fourth day of glitchmas my painter™ gave to me
four random crashes, three broken brushes, two system hangups & one corrupted workspace