AMD cards do not have this restraint.... 6xxx series cards are much better at it then the previous generations, and the 7xxx series are supposed to be better yet, but only time (and perhaps more mature drivers?) will tell.
to add to death's comment, it also has to have the same ram amount. I also prefer same brand/card
According to Nvidia's website you can use 2 cards with different amounts of RAM. This is copied directly from Nvidia's FAQ section on SLI.
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.