Overclocking
I used NVIDIA's overclocking utility, which is embedded in the ForceWare drivers and easy to unlock with the CoolBits registry hack, to overclock the cards. NVIDIA's overclocking utility is particularly handy, because it can automatically overclock cards and test new clock speeds to verify stability. Testing to verify stability is more important than you might think. Usually, one can fire up a game and look for artifacts or wait for crashes, but as id Software's John Carmack pointed out in HardOCP's Official DOOM 3 benchmarks, newer games like DOOM 3 utilize transistors that other games leave dormant. A stable overclock Far Cry could show artifacts or suffer from instability problems in DOOM 3.

In the interests of finding the most stable overclock for what's hopefully a wide variety of games, I relied on NVIDIA's automatic overclocking utility to detect the highest stable core and memory clock speeds. The automatic detection routine was run at least three times to ensure consistency.

In testing, I was able to get the following core and memory clock speeds from each card:

• e-GeForce 6800 Ultra - 464/1180MHz
  
• BFG GeForce 6800 GT OC - 439/1130MHz
  
• PNY GeForce 6800 GT - 421/1140MHz
  
• Chaintech AA6800 - 380/811MHz
  
• inno3D GeForce 6800 - 375/798MHz
  

For the eVGA e-GeForce 6800 Ultra, overclocking doesn't have a huge impact on frame rates. The card is already a performance leader, and cranking up the clock speeds a little doesn't change that. However, the overclocked GTs come very close to equaling the overclocked Ultra's performance. Our GT overclocks were good for an average frame rate gain of about 10 frames per second, which is quite respectable.

On the vanilla 6800 front, overclocking can have a huge impact on performance as long as you keep the resolution reasonable and antialiasing and anisotropic filtering disabled. Overclocking still helps at higher resolutions and with 4X antialiasing and 8X aniso, but the frame rate boost is less pronounced.