I could knock off about 20c-30c in my case in use since 2003. It doesn't have that many holes in it so sealing it up isn't too hard. Plus it has a layer of foam rubber insulation on the inside to keep the heat out plus deadens any noise from any fans inside. The video card output air is directed right back into the case, don't want to loose any cool air plus there is no heat output into the room
. Sounds weird but still using the same 5000 btu air conditioner from Wal-Mart. Currently blowing 8c to 14c air into the case and the air in the case goes right back to the air conditioner in a closed loop. A two year old card comes out as clean as when it went in since the air is relatively enclosed with a filter on the air conditioner. Made this case for a 9700 Pro from Tyan, next a 6800 GT followed by a 8800GTX, 260GTX and now a 5870. My 5870 overclocked at 950mhz never goes above 50c with the heaven benchmark, more like 46c without upping the voltage, still don't know how high I can push my 5870 but I am so cpu limited it becomes pointless to go further. Going below freezing as in -6c I've done but when I go too cold the hard drives have problems plus the fans start making noises (I figured the grease or lubrication was hardening up). Not only does it cool the GPU but everything else as well so overclocking ram, CPU is usually rather easy. Noise levels are rather low since fans don't really need to turn that fast. The down side is a higher cost in the electric bill plus some ducting and an air conditioner.
Gigabit DSL, Q6600 @ 3.4ghz, Mushkin 4gb DDR 800 OCZ 2x2gb , PowerColor HD 5870. 24" Acer LCD, 1TB hd with Vista 64 Home Premium.
Foxconn 780G, Athlon64 X2 5600 @ 3ghz, Mushkin 4gb DDR 800 OCZ 2x2gb , eVga 260 GTX. 17" MagAcer LCD, 120gb Vista 32