PSU:
I can safely say I won't trust your older PSU for the GTX260 or GTX280. Even if you got an adaptor to plug the TWO pci-e connectors that those cards require, the PSU is not rated for the kind of power that a GTX 260 or GTX 280 require.
Your PSU review, check the wattages of the rails:
Antec NeoPower 480 PSU | silentpcreview.com
GTX 280 review, power consumption:
GeForce GTX 280 review
I'm going to assume that your GPU will draw the power from the +12V2 rail, rated at
192W. According to the review, a GTX280 draws
~236W, which even exceeds the power of the main 12V rail of your PSU (216W). Given those figures, your PSU will overheat and shut down, or not boot at all with a GTX280. Add to this the fact that your PSU is "older" and my not supply its rated current anymore, and your desire to OC a quad core on it (add on another
~160W from overclocked FSB and CPU).
All in all, your computer won't start or won't last long. :(
CPU:
What price are you getting the Q6700 for? IIRC the performance between a
Q6600 vs Q6700 isn't much (it's only one multiplier step), and a Q6700 is much more expensive than a Q6600. I'll leave the CPU recommendation for someone who owns a recent Intel rig, but I'd probably use a Q6600 or less expensive CPU and put the extra money towards a new power supply for your hungry graphics card.