If you're a tinkerer, your best bet would probably be a G5 PowerMac (don't go for any of the lower-end ones, the high end ones are much more reliable/long lasting). Obviously you could always go for an older MacPro, but these will be much more expensive.
Personally though, I'd say your best bet is to get an Intel machine, particularly one of the new iMacs (which are absolutely brilliant btw). When I was a PC user, I used to swap bits of hardware out all the time, but now it's just not something I want to do. With Macs, I've found you tend to find yourself doing much more productive things than swapping out graphics/sound cards etc.