First of all, DDR3 will not make your computer any faster. It will only allow the processor to reach a higher clockspeed. Your goal is to spend the most you can on the processor and minimize wasted money on the Ram and motherboard. Neither actually effects the performance of your computer build.
looks great. That power supply
will survive more than just a few upgrades, as long as you don't plan on using more than two video cards or an insane amount of harddrives, you shouldn't have to replace it for a while. As far as ram goes, I would recommend this
As long as your gaming you won't use more than 4gb, so getting two 3gb sticks would be a waste.