go with the 560 ti. slightly lower performing, much better power and heat output. you can OC the difference anyway. the 560 is a newer card and is currently getting driver support in the latest games, not to say the older card isn't compatible or whatever. but it never hurts.
like the other two said, 560 TI for sure.. the 480 is comparable to the GTX 570 but runs way hotter and uses way more power then the 570. the difference you would see in performance between the cards at that resolution are minimal.
Agreed, the GTX560Ti is slightly slower, and doesn't have the grunt for higher resolutions that the GTX480 does, but at 1600x900 you'll never notice the difference, and for the sake of a good 60-70W (and a lot of heat and noise) saved, the 560 is the better card for you, even if it were the more expensive I'd still say take it over the 480.
Same 560 Ti, overclocked a 560 Ti will be faster than a GTX 480 and slightly faster than a GTX 570 which is about equal or 5% faster than a GTX 480. It also consumes less power, runs cooler, has a quieter cooling solution etc...
You can overclock the GTX 480 to surpass a overclocked GTX 560 Ti but the caveats outweigh the benefits IMHO.