666 gpu core clock?
Seen the EVGA GTX260 FTW clocks?
I recommend just going with an HD 4870 and a Gigabyte P45 UD3R to improve your cpu oc and eliminate any bottleneck. Then sell your 680. I would.
There's nothing wrong with the 680 for low-mid OC's, it's a decent performer and if he wants to go SLI he's set. As-far-as performance he won't gain anything other than a higher failure rate, crappier drivers and a higher operating and idle temperature with the 4870, the 1GB variant would be the closest match to the 260. Really either way would work, just depends on what one would want to go for. Don't get the 48xx or 260 because of the hype get it because it fits the bill of what you want and what you need, and how long you plan to keep it. Do some research and make the choice that suits your needs...I love my GTX it's a great card, many others are catching on to that fact as it's performance is very strong, it runs pretty damn cool, and flat out gets the job done. So does the competitor, but imo it failed in too many areas to warrant me spending MORE money for a product that was too close in performance with more issues. Things have changed on both sides since then (around mid-july), but performance is still close, and it really comes down to what you want.
As-far-as overclocking, I've seen many 680i's go beyond the 400FSB range which at 400 lines up perfectly for 3.6Ghz on a Q6600 which is MORE than enough for modern gaming...I had one at that speed, and it performed great, but I gained little to no gaming performance beyond 3GHz, and really gained maybe 1-2FPS between the stock 2.4Ghz and 3.6Ghz (when gaming at 1440x900 and 1680x1050) with a small range of an x1950xtx, 9600GT and GTX260. Overclocking the GPU is far more effective in gaming than the CPU and system overclocks are overall. Sure it helps to have some system overclocking, it will help in the future with games that take advantage of more than one core (which more games are doing now...), but if 4 cores at 3GHz can't handle a modern game...odds are it's either an unoptimized mess or way ahead of it's time.
Having a good and OC'd gaming rig on a many budgets is more about mixing what you want and need, a little excess never hurts because as-long as you keep the rig stable and in-line odds are it will be around longer if you're not upgrading every other month, which can be a nice payback to a good investment.