Just looking up the specs on a 550Ti. There is only a listing for the one. It operates at 900MHz (core). So yes, Asus has some models OC'd a bit.
I also see the 550Ti is rated at 691 gigaflops (graphical processing power). That is pretty weak. For example: a 650Ti is rated at 1420, which is more than double. And the GTX 680 is way up there at 3090 gigaflops. Considering the graphical power advantage a 650Ti has over a 550Ti, you may want to second guess going SLI. Especially when you consider a 650Ti isn't much more $ and uses 6 less watts (110 vs. 116).