interesting problem. as i dont have any exp with SLI..i can just guess
try updating ur drivers of the mobo and gfx cards.
make sure the bridge thing(not sure wat its called) connected properly
make sure both the gfx cards are not on a single power cable.
if all else fails. try using only 1 gfx card and c if it works..try running some benchmarks on it.
also the drivers have to be configured for each game for sli to work properly (auto setting r there..even so try it out).
as you have a 550 watts smps..wat are the other things you have on your system (no. of hdds, fans, LED

)