Personally, I'd go with the 6800GT card. Then, you could upgrade your setup in the future with another 6800GT card.
You buy two 6600GT's right now, and you're stuck. 6600GT's are good for people who *can't* afford the 6800GT, but want the option to upgrade their performance later...
Then I take it, you are going Nforce4 S939. Right now, you can get 6600GT's in PCI-Express, but good luck finding 6800GT's with PCI-Express. I would agree, with the above post, I would rather get 1 6800GT now, and get another later. That is the same situation I had with hard drives. I was going to get 2x36 GB Raptors in RAID 0. But then I realized the 74GB version was a lot better. I was going to get 1 74GB Raptor and get another one later to make a RAID array. It just so happened I got a really great deal on the 74GB's, so I got 2.
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