I dunno what your budget is, but I'd suggest a CPU running at least 1 Ghz, and at least 256 megs of DDR ram.
...
You're better off getting a custom-built one, even if you don't want to do it yourself. You won't save much money (in my experience) but you will get a PC built with top-quality name-brand parts. ~ Use only retail/name-brand parts for the motherboard, videocard and soundcard. -Get "real" parts, don't get parts "based on the same chip" as the good part. Avoid motherboards with built-in sound, video, firewire, NICs or whatever else. Avoid name-brand computers such as Dell, Gateway, IBM, Sony, Compaq, H-P, any low-end generics, and so on- they tend to use generic parts, usually a generic/proprietary motherboard that often is not strictly ATX standards-compliant and that can result in unfixable problems later (-the BIOS's on these boards often don't have all the control options they should-).
~
If you decide to build one yourself, the motherboard should come with instructions telling how everything else goes.
Also, if you buy the motherboard/CPU/memory all-together from some places, they will test it before shipping for a small fee. Pay the fee and have them do this.