600-1000 gaming build

Approximate Purchase Date: This week (i.e. the instant I commit to a build)

Budget Range: 600-1000, I'd like to be as low as possible unless the increase from 600-700 to 900-1000 is hugely massive performance wise for me.

System Usage from Most to Least Important: Gaming: SC2, and also RPGs and FPSs (think Diablo III, Bethesda RPGs, CoD, Quake etc)

Parts Not Required: Mouse, monitor.

Preferred Website(s) for Parts: Newegg.com

Country: USA

Parts Preferences: I'd like to find a nice mechanical keyboard. Is there anything better/cheaper than the Razer BlackWidow? (Love my Razer DeathAdder mouse).

Overclocking: Maybe

SLI or Crossfire: Maybe

Monitor Resolution: 1280x1024 (I'm using a 19" Insignia LED-LCD TV/Monitor. This is good for gaming, right?)

Additional Comments: Now, my current monitor has a low resolution so I feel like if I did a build based around an i5-2400 and a Radeon HD 6870, I'd be in pretty good shape to run most games at a really high resolution. One thing that I am considering is building a more basic rig around these two things (similar, I guess, to the 600 build-off challenge posted here) but to instead get a motherboard that will eventually let me do dual 6870's, which would mean that I'd be boss later if I got another 6870 and a better monitor but would let me have an awesome experience until that point, if it ever comes.

I'm also unsure as to whether 4gb of ram is sufficient, or if I need to go for the 8gb.

I'm really excited about building a computer and this site has been a massively awesome resource.

I'm upgrading from the current-gen MacBook Pro (the highest-end 15", which I've not been happy with - it was an awful decision given that I could literally have built an absolutely perfect high-end PC or gotten a really nice PC and a freakin' laptop for still less than the price that I paid) so I don't have massively high standards.

Note that the price of the OS will need to be factored in to my build.

Thanks so much.