1GB of VRAM @ 1080p will be OK.
it's the GTX 550 Ti as a single card can't run 1080p in gaming well.
and 550 Ti in SLi is a waste to be if your buying new retail.
a GTX 550Ti @ 1080p is not gaming, it's frustrating...
Anyone that buys SLI or CF setups like the 550ti or 7770 are just kicking themselves in the teeth.
Sell the one 550ti you have and get a better single gpu.
a 670 will do excellent.
Basically a 550ti in SLI performs right under a 570, and a 670 performs right under two 570s in SLI.
So a 670 is 4x faster than a single 550ti if you go by that.