My RAM choice for E6400

Depends whether you mean "overclock of my memory" or "overclock of the CPU this memory will sit beside." In case 1, very little; in case 2, you could do pretty well.
Synergy6
 
If you can..take it back! Most, if not all 965 mobo's won't let you run your ram less than 1:1 at this time. So if you plan to take your motherboard to anywhere close 400fsb then your ram must be capable of ddr 800(pc 6400) speeds. I seriously doubt that ram can handle it. 10-15 more will get you a fast 1gb set that can do 400fsb. And, if its in your budget 2gb will be helpfull if your thinking vista, but don't sweat it if you don't have the money