If you're eager to drop a lot of cash now or wait till later then by all means build away!
But I just recently put a GTX650ti in my system and with 4GB of ram I run BF3 and CoDBC2 at 720p with decent settings on my 32" screen and it is smooth so I am sticking my extra cash in my jar for my next build which will beat what I could buy right now!
Plus, I might even be able to jump to the newest tech at that time, depending on when I feel I can't play a game I want to play at a decent quality and frame rate.
The E8400 can OC very well if you have the right mobo and heat sink so you can squeeze more out of it. I had mine up to 4GHz with stock but trimmed it back as the heat would have killed it and I could not afford the after market heat sink.
So there you have it...hope this helps!