Overclocking the GF 7800 GTX 512MB
For our next set of performance metrics, we spent a little time overclocking our matched pair of GeForce 7800 GTX 512MB cards using the clock frequency slider available within NVIDIA's Forceware drivers after enabling the "Coolbits" registry tweak.
We'd like to note that overclocking video cards in SLI mode is somewhat more difficult than overclocking a single card. When overclocking a pair of cards in SLI mode, your peak overclock will be limited by whichever card overclocks the lowest. If card A's core can hit 600MHz, and card B's core can hit 590MHz, both cards end up being clocked at 590MHz, even though card B still has some clock speed headroom to spare. Each card is not overclocked individually in SLI mode to alleviate potential synchronization problems.
Due to the fact that the GeForce 7800 GTX 512MB is based on the same GPU, built with the same manufacturing process, we weren't expecting it to have very much juice left in the tank, considering the 512MB card's GPU is clocked 120MHz higher than NVIDIA's original reference spec for the 256MB version. We ended up being somewhat surprised, however. In a single card configuration, we were able to take one of our GeForce 7800 GTX 512MB cards up from its default core and memory clock speeds of 550MHz / 1.7GHz, up to 597MHz / 1.78GHz. For those keeping count, those are core and memory clock speed increases of 47MHz and 80MHz (DDR), respectively. Not too bad.
Our results while overclocking a pair of 512MB GeForce 7800 GTX cards in SLI mode were very similar. With two cards running in the system, we hit peak core and memory frequencies of 593MHz and 1.77GHz -- also, respectable increases. While the card were overclocked, we re-ran a couple of benchmarks to see what kind of performance increases we'd find. As you can see, performance in 3DMark05 and Doom 3 increased marginally while overclocked. Perhaps if our test system was equipped with a faster CPU, the performance increases while overclocked would have been higher.