Well to reitirate what kajabla said...If you got the cash go for dual 590 GTX's or dual 6990's....I would opt for the 6990's in that scenario..Otherwise please give specifics on your set-up, budget, and use.
Well the OP said $300..So thats why I stated that about the 6950, I found a 570 at the EGG on shellshocker the other day for $299.00....Yeah I'd probably opt in on the Nvidia card too, maybe...I am partial to AMD...as I use AMD CPU's I like my components to always play together proper...I was one happy camper when AMD purchased ATI !!! I always stick to: AMD CPU's with AMD GPU's...Intel CPU's with Nvidia GPU's
^Nice sentiment, but it really doesn't matter. All GPUs and CPUs play nice. The same isn't true for the games - TWIMTBP games, like Crysis and Just Cause, definitely favor Nvidia cards, and the same goes for some AMD-sponsored games, I think.