DDR2 and DDR3 aren't interchangable, so the answer is no. The antec 550 ps is marginal for your system; what video card do you plan to use, or will you use the onboard video?
atm onboard video is what im planning on using not for too long though. When i do get a new video card it wont be top of the line. Could you recomend any power supplies then? how many watts am i going to need?