what OS are you planning on using? If you're planning on win98, keep in mind win98 does not work well with more than 512ram.
I recently upgraded my computer with an AMD 2500 after reading that this chip OCs very well.
Choose your graphics card carefully that will make probably the bigest difference in your gaming.
Keep cooling in mind also, choos a case that alows for quality cooling, don't get a small case where the power supply will be right next to the processor.
It will be worth the money to get extra cooling fans if case does not come with them. (at least one in the front and at least one in the back).
You will also want a quality PS most cases come with cheap ones that don't last long.

Thats my opinion so far for this thread...