Gpu not working properly

hello guys ... I'm probably very new to this. I used to have a sparkle geforce 9500 gt gpu. I've decided to upgrade my gpu as it doesnt support the latest games.I've purchased a zotac gtx 550ti igb gpu.Since then my problems have started.The latest games were not performing well at all.There is lagging everywhere and it is getting stuck evrywhere.Games like f1 2011 gave 17 avg fps(on low setting), nfs the run gave 8-9 fps(on low setting) etc., I thought my old psu(frontech 450w) was the problem and bought a new corsair cx600 psu. Even then the situation didint change. The new gpu is not even performing like my old 9500 gt. I'm a gaming enthusiast and I'm not satisfied with my gpu perfromace at all. PLS PLS PLS PLS help me out guys....
My system specs are ...
Intel dg41rq motheroard
intel core 2 duo E7500 2.93 ghz processor
2 gb ddr2 ram
Corsair cx600 600w psu
Zotac nvidia geforce gtx 550ti 1gb ddr5 gpu

I heard that u guys are the best in solving problems .... pls pls pls help me out