The 550 Ti is just a so-so card. Although I don't know your budget or your purposes, it makes more sense to spend the $15 on just moving up to a better card, period. The VRAM is a secondary concern compared to the strength of the GPU itself. Assuming you are in the US, consider either of these 560s, which match approximately in cost the 2GB 550 Ti after rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16814500225
I can't imagine a situation, even at e.g. very high resolution, where the 2GB 550 Ti would outperform a 560 1GB.
edit: here's a decent sense via a good all-around benchmark; you can see the 560 is easily 40-50% faster in many games: http://www.anandtech.com/bench/Product/541?vs=543