A good cpu for multitasking and gaming as well will run about $170 by itself. If you shop around for a bundle you usually see one of the lower models of that socket type going with the board selected. For upgrading a current build you still have to consider memory and the video card used unless it's one of the latest models out.
Here I do everything with an AM2 6000+ X2 since the 6400+ wasn't instck when ordering for the new build here. It does quite well for a stock setup with a mid range ATI model card while the first thing you will hear from most is strictly Intel and NVidia. You'll want an nForce chipset over VIA, SIS, AMD even, and Intel is out for ocing.