Comp upgrade

I built my comp with this sites help a couple years ago now I'm looking for a quick upgrade, here are my current specs

win7x64
cpu - i5 750 @ 2.67
patriot viper2 1600mhz 4gigs ram
ati 5750 1gb ddr5
asus p7p55d pro motherboard
seagate 1tb sata 7200rpm
corsair 750watt

I'm just looking for a quick and easy upgrade, not to sure how to overclock yet but i would be willing to try the cooling system is just stock tho thats why im afraid to try. Also only reason im upgrading is because i mostly play mmos and i thought this would handle it for awhile but it only runs at about 15fps in big 20man raids. Any help would be appreciated.