i would be happy with that the 4870 is still a decent card ....

at some stage you will need to go different mobo and processor so might aswell do it now....
mate of mine built a 920 rig a while ago and on games it flies along nicely even with a mid priced gpu ....
so with a 6950 or a 6990 and what your already looking at should work well .. lol.. been looking at the weekend and i think i need a better job or start saving ....