2x GTX460's will easily outperform a single GTX480, but later when you want to upgrade you will be stuck with both of your gtx 460's and be forced to throw them both out in order to do the upgrade, while if you buy a gtx480 then later when you want to upgrade you can buy another gtx 480.
your power supply is more than enough and since you don't plan on upgrading anytime soon, then the gtx 460's in sli is your best choice since it even outperforms the 5970 in some games and it have a better minimum fps as well.
Alright, thanks for all of the feedback.
One last question. Which 460s, and Is the ud5 a good mobo to stick them in? I'd like to have my soundcard in use too. Is that going to be possible? (basically whats the best mobo for dual 460s (if Evga are the best 460s, should i go with a FTW3 or something?)