Well it all depends on what your price range is.
If your looking to play games, your going to want to have a fairly sufficient ammount of RAM, and a high end video card. If you can find a pentium 4 that supports hyperthreading your going to want to go with that, for a semi-strict price ranger, otherwise spring for a solid dual-core. Intel Core 2 Duo are faster than hell if you ask me.
Basically look for something along these lines.
1gb RAM
120gb @ 7200RPM or 80GB @ 10,000 RPM for gaming (going to cost extra)
128MB GPU (Video Card) or 256MB+ for gaming (going to cost extra)
3.0Ghz Pentium 4 intel w/ Hyperthreading technology, Any core 2 duo, or A mid to high end dual-core processor (2.2Ghz or higher)
Keep in mind you dont want anything to bulky and if they try and get you to spring for a sound card, dont bother the onboard one will be just fine. If you have any questions im here.
~System-Root