Well, GTX 560ti is really good but the HD 7850 is a good value for the money and almost performs as fast as GTX 570.
Any card of those will max out D3, WOW prefers a faster CPU than a faster GPU.
What's your gaming resolution?
Considering that you'll change GPU,PSU, CPU why not take the move and upgrade the whole system to SB or IB platforms? the main problem is that your OEM doesn't allow overclocking, so you won't get the most out of the CPU.