if you wanna go for dual-card called sli you need a motherboard with dual pci-e slots then you will be abe to do.
by the what is your full specs:
I don't think he is talking about running SLI on the system. I think that he just wants to replace that 450 with a 560ti which is a two-slot card.
Are you saying this is more of a computer case issue where the slots on the motherboard is so small that it won't line up well with the case's addon slots?