Hi Randomaxess,
Most heatsinks that you see on video cards processors are epoxied on, so screwing on a small 40mm or 50mm fan on to the heatsink won't really be a problem. But since you have the slot fan below the video card, I think you won't have to worry about your video card overheating.
If you are trying to cool the video memory, you could epoxy on small heatsinks on them. I've seen Alpha Wing Fin heatsinks and video card cooling kits available from 2CoolTek.com. He also has heatsink epoxy available.
http://2cooltek.safeshopper.com/51/cat51.htm?515
The Antec SX830 is actually a workstation/server case and was designed with good ventilation in mind. As long as you install 2 intake fans in the front and 2 exhaust fans in the back you shouldn't have to worry about heat build-up in the case. I think cutting a hole for a 120mm fan would really be overkill.
You say you want to overclock the video card...are you overclocking your whole system as well? If you are, please go slowly and in small increments.
You may want to check out this Forum as well:
http://arstechnica.infopop.net/OpenT...562&f=77909585
Cricket