No the keplar core doesnt matter. To SLI you need to do it with 2 of the same cards. They dont need to be the same brand though. Meaning you can SLI a MSI card with a EVGA card. As for the 650, if your still wanting to use it. Use it for a dedicated physx card if you play games that support it, and I dont think the 650 even has a SLI connector.
Alright thanks! how would one set this up though? Link maybe?