The best bang-for-your-buck cards out right now have Nvidia Geforce 6600GT chipset in them. The card comes in both AGP and PCI Express x16 versions. I've seen them as low as $119 after rebate. I've built a couple systems with this card and although they aren't stellar performers, they will run most of the new games at 1024x768 with low to medium settings. It ran the Reflex XTR Demo on a P4 3GHz w/ 1 gig ram at 1280x1024x32 very smooth with v-sync both on and off (Powercolor 128 meg AGP version).

If you tell me how much you want to spend, I'll tell you the best ATI and the best Nvidia card you can get for your money.