The 8600 cards suck. The HD3600 cards suck too, but at least they provide hardware-accelerated video codecs free.
While the 9600GSO does start at like 110.-, there aren't many choices.
What annoys me is that both Gigabyte and ECS offer fanless 9600GTs, but nobody offers a 9600GSO that way, despite it likely being a lower-heat part. (I'm a big fan of fanless video cards)
In that spirit, how about
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121237 - a fanless GDDR3 HD3650, about 70 bucks after rebate.
I've heard that in the end, SLIing two weak cards usually ends up not delivering huge bang for the buck. After all, you're paying a premium (or at least narrowing your choices) picking a SLI board, on top of the second card.
If you can save $30 on the board by not going SLI, that pushes your "second card" budget from $100 to $130, enough to get an 9600-series card when you're ready to upgrade the board, and sell the old one on Craigslist for 30 bucks.