I think you should keep what you have, 8gig of ram is very good even if it's DDR2.
Just a simple O.C. of your prossesor would be easiest.
It looks like you want to buy a whole new computer basicly,my reccomendation would be to save some more money for a new build later on down the road and just get a single graphics card for now and then on your new build crossfire/sli with the graphics card your going to buy now.
What type of power supply do you have(make/model)?
And what is your budget for this?