NVIDIA GeForce 7800 GTX

Overclocking the GeForce 7800 GTX & 7800 GTX SLI (Fastest 3D Video Card x 2) / SLI + Overclocking = Geek Euphoria

As we neared the end of our testing, we spent a little time overclocking our matched pair of GeForce 7800 GTX cards using the clock frequency slider available within NVIDIA's Forceware drivers after enabling the "Coolbits" registry tweak. Overclocking video cards in SLI mode is somewhat more difficult than overclocking a single card, however. 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 475MHz, and card B's core can hit 515MHz, both cards end up being clocked at 475MHz, 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 (although this may not be the case when the Rel. 80 drivers are released)...

Single GeForce 7800 GTX OC=471MHz / 1.28GHz  |  GeForce 7800 GTX SLI OC=463MHz / 1.26GHz

Single GeForce 7800 GTX OC=471MHz / 1.28GHz  |  GeForce 7800 GTX SLI OC=463MHz / 1.26GHz

We've got two sets of overclocked numbers to discuss here: the single GeForce 7800 GTX overclock and the SLI configuration overclock. The scenario we explained above played out in our overclocking experience because one of our 7800 GTX cards overclocked to a higher level than the other. In a single-card configuration, we were able to take one of our 7800 GTX cards to a maximum, fully-stable core clock speed of 471MHz, with its memory clocked at 1.28GHz DDR - clock speed increases of 41MHz (9.5%) and 80MHz (6.6%), respectively. In an SLI configuration, however, the highest core and memory clock speeds we could reliably hit were 463MHz and 1.26GHz.

With the cards overclocked, we had only marginal increases in performance. The single GeForce 7800 GTX card's 3DMark05 score jumped 415 points while overclocked, and its frame rate in Doom 3 (1,600 x 1,200 - 4XAA / 8X Aniso) climbed by a couple of frames per second. We saw slightly smaller performance increases with the SLI configuration, due to the lower overclocked speeds and affect our CPU had on performance (a pair of GeForce 7800 GTX cards in an SLI configuration is more CPU bound than a single card in these tests). The 7800 GTX SLI rig's 3DMark05 score increased by 242 points, and its Doom 3 frame rate went up by 3.7 FPS.

During our overclocking experiments, we also monitored the core temperatures of both of the GeForce 7800 GTX cards we used for testing. While in a single-card configuration, with an ambient room temperature of around 72^oF (22.2^oC), core temperatures hovered around 49^oC when idle. Under load, core temperatures climbed to 79^oC. Temperatures were similar with the cards in an SLI configuration, but due to the proximity of a second card generating an equal amount of heat, they were a bit higher. While in an SLI configuration we saw core temperatures that hovered around 53^oC (idle) and 82^oC (load).