SLI Cards

I'm thinking of buying a new video card, but I would like to probably just add one in SLI mode. I have an EVGA 8800GTS 320MB. I believe I read that I do not need to get the same brand, but do I need to get one that has the same amount of memory or can I get another brand 8800GTS with 512MB (or more) of VRAM?

Any thoughts if this is worth it or should I just get a whole new single video card to replace it? Some of the 8800gts's I've seen on Newegg look pretty cheap.

well now days the 8800GT's with 512 are very cheap on newegg.com.. some are $110

and no you cannot use yours, and a new one in SLI.. they wont work. you need to get another one like yours.. you can get either or, a 320, or a 640.. but since one is slower.. they will both run at the slower ones speed of 320. but thats ok.. so either do that. but will be hard to fine. or just get one new 4870, or a GTX260