I can answer you question as ive just done exactly what youre thinking..
I had SLI 480 and swapped them out for a single GTX680.
Gamewise performance is pretty much the same, ive not noticed any difference in the games i play - MW3, BF3, Metro 2033 and BL2
I got £180 for my 480s and paid £372 for my 680 so the change cost me just shy of £200, but i had the cash to spare and had the upgrade itch
So heres the pros and cons of the upgrade :
Pros :
1. Much less heat. Card runs really cool, maxxing at about 60C under load.
2. Much less cluttered in my case and improved airflow
3. Machine uses a lot less power. Im saving about £4 a week in electric ( prepay meter thats monitored daily )
4. Nice feeling from having a top of the range card
5. More VRAM - may be useful in the future.
Cons :
1. Cost. In hindsight not really worth the £200 it cost me.
2. No increase in performance compared to the 480s
So in summary, if you can afford it then go for it, but you wont see any increase in performance, but will get a cooler running and less power hungry machine
