For future reference, this should probably go in Computer Tweaks and Setup.
I would go no less than 2.0GHz and 1GB of RAM. Be aware that you will need to match the processor with the chipset on the motherboard, and also the RAM type and speed with that of the motherboard (you want to get all the performance out of the parts you buy; don't buy slower RAM than the motherboard will take). I recommend DDR400, or DDR2 if you can afford it.
As for processors, don't use Intel Celeron. I don't care how "fast" it might be on paper. You might try AMD Opteron for a low cost, decent solution - solid multitasking abilities. There is always the Intel Pentium4 if you like raw speed, or the AMD Athlon is also a good, or Athlon64 (if you want to play around with running a 64bit OS now or in the future).
Personally, I would go with an AMD Athlon. AMD runs much cooler than the Pentium4. That said, you want a good CPU cooler regardless. Look into *copper* (not aluminum) Thermaltake or CoolerMaster CPU coolers. This is important - I had a P4 2.8GHz running with the stock aluminum cooler at almost 60 Celsius *idle*. I could *feel* it lagging. Put a copper CoolerMaster with heatpipes on it, and it dropped to 43 Celsius. Runs like a dream. Heck, it boots almost twice as fast.