I think only in certain situations you can sli cards of different Vrams, but its generally something you shouldnt do(as the higher vram card would be wasted as it can only be equal to the 1 gb vram model)
Like dudewitbow said sli'ing/crossfiring two cards with different vrams keep in mind the higher vram card will be automatically level itself with the lower vram card so higher vram is just totally waste I wouldn't recommend to sli those.
Can I mix and match graphics cards with different sizes of memory?
When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. However, while it is not recommended, NVIDIA does offer the flexibility to run graphics cards with different sized memory by using CoolBits. Using CoolBits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate them together in SLI mode. When dissimilar memory sizes are enabled to work together using CoolBits, the effective memory size for each card becomes the smaller of the memory sizes.