The amount of video RaM is important only in that you need 'enough' framebuffer size for the given resolution and textures/details to be processed/displayed and levels of AA/AF to be processed; having more Vram than needed does not make the card faster at a given resolution. The 550Ti has fewer CUDA cores and a crippled memory bus relative to even the older GTX460(768mb) cards.
(Generally speaking, 1 gb is 'enough' at 1080P, and 1.5 gb is usually enough for 2560x1600, with 2gb nice for 3 monitor gaming with effective 5700x1080 res)
The 560Ti and 6950 both offer very good performance for their $200/$220 pricepoints...