It really doesn't take as much out of your resources as you think. It doesn't make a big difference unless you have like 64mb of ram or something.Just make sure you disable the bloated GUI (I belive it's name is "Luna"?). Stick to the Windows 2000 look and you'll be ok. You should be running at least a 1.5GHz machine in order to have the new GUI working properly without taking precious cycles from the CPU.
It can run fine with 128MB. Of course the performance is better with more RAM, but I used XP on a 350mhz machine with 128MB of ram for half of this past year and didn't have any problems with performance except with the obvious things like 3d games, photoshopping, 3d studio max.And the most important, HOW MUCH RAM?? XP can't really run properly without at least 256MB.