Building new PC - What parts?

Hi!

Im planning on building a new PC which i would be using for gaming and and basic things like web browsing, music, video etc..

Gaming is first priority. I need some help to get some parts which will be able to run games from here on until a certain year. I dont want it to be after 6 or 12 months i have to upgrade the whole build, but if its just installing something small like extra HDD or Bluray drive that's fine.

I really want to play Battlefield 3. If i can play this at very high settings i should be able to play other games right? Crysis?.
the highest setting for Battlefield 3 is ultra high. Whats recommended for ultra high?

Also i come across threads on forums talking about Crossfire or SLI, thats 2 GPU's right? Im quite a noob at computers right now but im learning haha. So try to explain somthing which i can understand.

I been looking at hardware and i dont know which ones to get. For example the GFX cards ATI6950 2GB and there's a 1GB model for it, then theres GTX560 Ti 1GB.