The 6800 GS is a $180 card, the 6600 GT is a $110 card ($139 in AGP version). If they are charging that much more for an SLI 6600GT it is not worth it.
With dual SLI Many games will NOT show a significant improvement. Even at best expect only a 50% gain for going to 2 cards.
http://www.tomshardware.com/2005/12/02/vga_charts_viii/page20.html
The 6800 GS is 12 pixel pipelines and 256MB RAM vs 6600 Gt at 8 pixel pipes and 500 mHz. You can get a 6800 GS clocked as high as 490 mHz for about the same price as the standard 425 mHz 6800 GS.
http://www.newegg.com/Product/Produ...&description=&srchInDesc=&minPrice=&maxPrice=
http://www.newegg.com/Product/Produ...&description=&srchInDesc=&minPrice=&maxPrice=
Having said all that I would not go for either. The improvement is not enugh to justify the added cost. Save up for a card that has not been crippled like the GS has swapping clock speed for pixel pipes and shaders.
If you do go SLI-
You did not mention it, but I assume the 6600 GT you already have is SLI capable right?
The two 6600 GT's need to be identical. Same memory clock speed, same GPU clock speed, same RAM size.
Be careful buying different brands. Sometimes the manufacturers run at different clock speeds with different models. It is recommended you run 2 of the same card to avoid compatibility issues, but not specifically required.
Another consideration. Resale value on a 6600 GT is $60 or less. That means your 2 6600 GT's will have less resale value than a single 6800 GS.
If my options were the 6800 GS or the 2-6600GT's in SLI I'd go for the GS. Leaves you with the SLI option open, resale value is better, power consumption is lower. And no "SLI hassles" to deal with (need config files for each new game, there may be compatibility issues, etc.).