My personal recommendation is for the X2 5200+, mainly because the Core 2 processors really shine when overclocked, and a good OC'ers motherboard for a Core 2 proc... well, let's just say there's not much "budget" about them. Not to mention that, once they come out, you can drop a quad-core Phenom processor into any Socket AM2 motherboard, so it renders the whole "do I have the right chipset in my mobo" argument moot.
killz86: Not quite sure what you're getting at there, but normally Intel's processors are far more sensitive to memory bandwidth available than their AMD counterparts.