Can I run 2xGTX 550 Ti on a Corsair HX-650?

Pretty much what the title says.
I've already got one 550ti, so I was wondering if I could just get a second one and run it in SLI.
my mobo is the Asrock 970 Extreme 4, so it supports SLI. my main concern is whether 650W isn't enough.
Also, what kind of performance could I expect from this SLI setup? I've heard it would perform similar to a GTX570.
any input is appreciated

Seeing how the card requires a 400W PSU and the card itself only draws 116W, I'd say an additional card would require only 520W. I think your 650W PSU will be fine for a Geforce GTX 550Ti SLI setup.

Easily. I power 2 Radeon HD6870's with my HX650 and those cards have a max TDP of 151 watts. The 550ti only has a max TDP of 116 watts. The HX650 wont even break a sweat. You will even be able to overclock and still have plenty of headroom. It is a very high quality power supply.

As for the performance, 2x 550ti's perform similar to the GTX 570.

Your PSU will be fine but I wouldn't bother with SLI'ing low end cards, a single more powerful card is always preferable as SLI is highly dependant on driver compatibility. GTX 480's are now selling for really cheap, just slightly more than a new 550Ti (which is an overpriced card for its capabilities IMO).