e8400 is a good cpu you will just be limited in games that utilize quads such as bfbc2 or bf3, the gtx 280 is about as powerful as a 6850/6870, id say keep what you have for now and save for a new build, there will be some interesting releases in the next few months.
The GeForce GTX 280 would fit in between the HD 5830 1024M and the GTX 460 1024M in the following relative performance chart. The Performance per Dollar chart is from May 17th 2011. Charts are from TechPowerUp.com's Video Cards Review site.
yeah its a tough call although If I did not have my gtx 570 I probably would be upgrading before the new gpu launches, probably november or beginning of the new year for amd and shortly after that for nvidia imo, but the performance gains to be had by next gens chips is all speculating at this point