i have one gtx 550ti which is 1 GB. now i Am getting second one .the card with 1 Gb Vram is more costlier then a 1.5GB one . so if i go for the 1.5 GB card , i'm sure i won't be able to SLI it. Can i anyhow do SLI with one 1 GB card and other 1.5 GB card ? HOW