Don't go with the LGA 775 socket CPUs as they are being phased out and there will be no upgrade options. For half the price you can get the Athlon II x2 250 or for about the same price you can get the AMD Phenom II x4 945. If you're really stuck an Intel chip, I would suggest the Intel i5 at $30 more.
For both of these options you'll need to get a new motherboard, socket AM3 for the AMD chips or socket 1156 for the i5. If you want a really good OC, I would suggest the i5, since it can be bumped up by at least 2GHz. *Reference*.