Most of the sims will actually work OK on even a 1GHz machine so if you get a modern one (2GHz .. 3GHz) the CPU of whatever you choose should be fine. But I would pay a bit of attention to the video adapter. Obviously it's got to be 3D and I'd get at least 64MB of video memory. No one ever got fired for buying nVidia based cards!

Personally I bought this a few years ago and even now it still gives great performance with almost all sims (but I recently upgraded CPU from 466MHz to AMD 2.4GHz):

http://www.prolink-usa.com/english/product/gts-64a.html

They obviously now have quite a range of much more modern (faster!) cards to choose from too. If they're anything like this one then they should be good.

Cliff