that's an odd connundrum - are you sure you mean GTX 550?
the choice is yours really but 2 x GTX 460 in SLI can be on a par with GTX 570/580 depending upon whether you overclock the cards.
The GTX 460 still costs more than the GTX 550 despite being an older card, look at this for a comparison:
http://www.hwcompare.com/9691/geforce-gtx-460-1gb-vs-geforce-gtx-550-ti/
so, 1 x GTX 460 is greater than 1 x GTX 550Ti
GTX 460 SLI is way better than 1 x GTX 550Ti (as stated before GTX 460 SLI can be on a par with GTX 570/580)
you need to have a closer look at your PSU specs to be certain it will cope with GTX 460 SLI, but at 620w I would hope so. Is your motherbard SLI ready and is the 2nd lane you are intending to use free from obstruction by anything else in the case?
chaz