Go for Core 2 Quad Q6600 (2.40GHz,1066FSB,L2:2X4MB,rev.G0,4 cores)
Check if its codename is "SLACR" - this is rev.G0 which overclocks better. Do not buy B3.
In order to overclock this CPU you simply need to increase voltage from 1.25 to 1.375 (in my case, can be different), set FSB to 400 MHz and reduce multiplier from 9 to 8. Also, set RAM to CPU ratio to 1:1 (FSB:RAM). And that's basically it ;-)
There are plenty of tutorials and guides on how to do basic overclocking and every motherboard should allow at last basic overclocking. You'll have to check it in the manual.
Oh, and don't forget about some nice cooling. Air cooling is fine for this configuration but buy something big. I'm using
Scythe Mugen and it keeps my rig steady even when doing days long rendering.