You are limited in what you can overclock for your computer. Big Box manufacturers want to preserve the computer so they limit a lot of the overclocking potential that you normally would have. They are afraid that you will ruin the system and call in for warranty.
You can try to make your video card a bit faster but not much. You can also overclock the processor but thats only if the option isn't disabled through manufacturer specific bios options.
Try getting into the bios to see if you can change anything as far as processor multiplier or hypertransport speeds. When you first turn the computer on at the beginning acer screen look for an option to press one of the function keys to get into setup. Let me know what options you have.
Intel Core i7 920 @ 4.0 Ghz
Coolermaster V8 CPU Cooler
Asus Rampage II Extreme
Nvidia GTX 280 Tri SLI
3 x 2 gig DDR3 Corsair
Thermaltake Toughpower 1200 W PSU