Well, it's not a real bad psu or something, been reading reviews and some people give it a low rating but a lot of others say it's ok. Anyway it's a cheap psu, not 80+ certified and normally cheap psu's don't have a very stable power output and the higher end cards need a stable input with lots of amps. So the 550 is fine, 560 also but the higher you go and the psu will fail i think.
I really think it won't be good enough to sli two 550 cards.
One comment from the reviews i've read ; Do not buy this if your trying to run 2 5700 or higher video cards, you will crash everytime.
i'm having second thoughts about Sapphire Radeon HD6770 ddr5. which is better for my system the HD6670 or GTX 550 Ti?