4GB (2x2GB) should be more than enough memory to run at that resolution. As I said it's higher clock speeds that'll give you the increases in FPS without going for a card with more shaders.
Gonna say it again cos I need to go sleep. Slow down a bit mate and before you order anything get on the phone to OcUK, you'll not only get sound advice from the forums but you'll get that from the staff there too; one of few decent companies out there that'll dish out honest advice and won't try and sell you what you don't need.
Also, just clicked on your profile and are you actually 12? I'm thinking that must be a mistake, seems a lot of PC for a 12 year old.