680i vs 750,780,790

Why do you want an SLI chipset when you are running a single card?

If you are stable at 4.2Ghz with an E8400 you should still be content. If you just have the need to upgrade wait 3 months and get a P55.

If you're stable then just leave it alone. 750i boards have fewer lanes. 780i/790i boards allow you to run two cards with two PCI-E 16x slots at 16x. You would see a slight increase with two PCI-E 2.0 16x lanes vs two PCI-E 1.0/1.1 lanes, if you were running the same two cards in SLI. For a single card configuration the difference is very very small. There is a small benefit to using DDR3 with the 790i board, but it's really not worth the cost. Also, if you get a new board there is no guarantee that it will be able to overclock as well as your current one leaving you worse off.

You're thinking of an upgrading to a newer generation chipset although 680i to 780i was possible 2 years ago. How big is your monitor and what resolution do you play in? Usually, upgrades are made to gain performance for a certain task. If what you have now is satisfactory, no lags in framerates or rendering times, then don't waste cash (or plastic credit) for a 2% gain that you won't notice.

I'm using an e8400 with a single GTX 260 on a 22" monitor after wasting cash on an SLI rig with 2 X 9800 GT's that doesn't perform as fast or smoothly as one card. My next upgrade will be more value minded to a core i5 that I will overclock and a 26" monitor. I'm expecting the GTX 260 will keep up but if I start seeing lags, then I'll wait to see what GPU's are available that will be a significant improvement. The GTX 275, 280, 285s aren't worth upgrading to. Maybe the GTX 295 in 6 months will cost less than $300?

IMO, upgrades that give less than 5 or even 10 percent improvement are a waste of cash and time.

The only reason I would change out what you have now if your happy with it is if your afraid of encountering the overclock bug. The 6xx series had a habbit of corrupting the SATA drives on overclocks. I believe a bios fix was released however, so you don't need to buy a new board to avoid it.

Swapping out the motherboard isn't really worth it. I would buy some things that will carry over to a new build. Buy a new case/PSU. Buy a nice chair. Perhaps you could use a new laser mouse, or upgrade your LAN to gigabit speeds. Uprading the motherboard to a new chipset isn't going to increase your benchmark scores that much. Buy something that matters while waiting for the next gen chips to come out. (matters and will carry over.)