I would get something approximately like this. I picked fast but this will give you the general idea.

Memory: G.Skill 4 GB 1066 $89.99
Motherboard: GA-EP35-DS3R $119.99
CPU: Q6600 $195

Total cost: $405. Plus you'll need a fan (edit: I meant cooler) if you overclock.

Early next year, you can use the $600 that you saved to upgrade again to Nehalem.