Well the GTS 250 1GB SLI performs a little worse then a single GTX 285, and a single GTX 460 1GB can overclock to higher performance then a GTX 285. To top that your getting the 512mb versions, not the 1GB versions. The GTX 460 is a single card so its less hassle and will have less hiccups. You will also have DX11.
Just my two cents.
You can also step up to a 470 in three months if you want, by getting an EVGA.
yea looking to go up to either a 460 or a 470 with Evga step up