With that budget, you would pretty much be forced to socket 754, and a Sempron 3000+ is about $75 or so..(roughly A64/2800 performance)
There are now socket 754 NForce variant mainboards ($65) with PCI-e video slots, which is a good thing....
Try to find a 6800GS for about $170, if into 3d gaming at all; if not into gaming, you could always use integrated video...
The above general selections assume of course you already have a case, a good ATX12v power supply (350 watts min.), and some 184 pin DDR modules waiting for install....