I have a 9800GX2 with a E8400 and I have about the same performance as a gtx 280.
Will you OC the q6600?
A Q6600 OC @3.0+ Ghz will have much more punch than stock... you'll need a CPU of 3.0 Ghz to get the max of the Video Card.
I also have a Q6600 for my server and it's OC to 3.0Ghz and I saw a very noticable gain in performance.
The Q6600 shines at or over 3.0Ghz.
And you should check the PSU to make sure it can support either the 9800GTX+ in SLI or a single GTX260.
Or you could wait and get the next gen card witch are just around the corner with DX11.
If you don't want to wait I'd suggest if you can you'll be better with ATI and CF.