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?
You can use up to a 4770 or 5770 with the 550 antec. If these video chipsets are adequate, then you should be fine with the antec. The 900 combo for $25 more with 600w ps is also a good price. If the noise from the 900 is too high for your liking, you can simply unplug some of the fans.