Odd. I had always thought mismatched memory was not an issue until a thread about a month ago where someone said that memory had to match after a certain nvidia driver (and all drivers thereafter).
I was surprised to confirm the info on (what I thought was) an nvidia website in the FAQ seqtion where they directed people to a program that would allow two cards with different memory to SLI.
The only mention I can come up with off hand is this:
http://en.wikipedia.org/wiki/Scalable_Link_Interface
"In cases where two cards are not identical, the fastest card – or the card with more memory - will run at the speed of the slower card or disable its additional memory. (Note that while the FAQ still claims different memory size support, the support has been removed since revision 100.xx of NVIDIA's Forceware driver suite.)"
This isn't where I saw it last month though...