I have several 8800GTSs (eVga brand at stock), one of which I have tried to cool in an old Sonata case. I originally had two 120mm fans in the case, one in the front drive bay pulling air in and a second in the back exhausting air out. It just wasn't enough. So, I used a Black and Decker recipricating hand saw with a metal cutting blade to cut a hole in the side panel for another 120mm fan. I installed a Stealth Vantec 120mm fan and I then used a "Style Guard" low profile door edge guard purchased at an automotive parts store to "seal off" the filed down edge of the resultant circle and capped this off with a fan grill I purchased at NewEgg. It doesn't really look all that bad. This kicked my idle GPU temps down to 58 C with a load of around 68-70 C. Trying to go lower I then tried both a "pull" and a "push" solution for a slot cooler. the "pull" solution was as follows:
http://www.newegg.com/Product/...x?Item=N82E16835888602
The push solution was:
http://www.antec.com/us/productDetails.php?ProdID=75060
I had purchased two of the Antec V-cools so I borrowed the extension tube from one of them to add to the second, such that I had two extensions. This allowed me to line up the cool air pulled in from outside the case by the V-Cool to be blown directly down the throat of the intake fan of the 8800GTS. I thought surely this will make a difference...well it kind of did...lowered by temps by about 1-2 degees C idle and load...nothing to write home about. I also tried the Dynatron slot cooler I purchased from NewEgg which is an exhaust fan, and it seemed to cool about the same as the V-cool at idle or low load, but lowered the load temps by about 4-5 degrees C. So for me the exhaust solution worked better. This kind of makes sense because the 8800GTS exhausts hot air out the rear but also has some open silde-slots towards the rear of the card that allows some of the hot air to exit into the bottom of the case.
The other GTS is in an Antec 640B case and this case is just marvelous. Idle and low load temps for the GPU are 51-53 C, full load at about 63 C. I have a side 92 mm fan blowing down the throat of the cpu from the side panel CAG tube, two 80mm fans, one in front of the hard drive, the other pulling in cool air from the lower side panel and a 180mm exhaust fan in the rear. Ambient temps range from 75 to 77 F. So, in summary, the most effective cooling solution is having an excellently designed case to begin with, one that moves a lot of air through it. Your case is a good one, certainly capable of moving a lot of air, which is a good start. I see people nickel and dimeing themselves to death trying to use slot coolers to make up for inferior case designs, only to achieve marginal results at best. Start out with a good case, and the rest is gravy.