Overclocking the Viper
For our next set of performance metrics, we spent some time overclocking the Diamond Viper HD 2900 XT 1GB using ATI Tool.
Please note, that overclocking is NOT supported on Radeon HD 2900 XT cards with the Catalyst drivers unless an 8-Pin PCI Express supplemental power connector is plugged into the card, along with a secondary 6-Pin feed. When asked why they made the 8-Pin connector a requirement for overclocking, representative from ATI said it was due to major variations in power consumption between not only stock and overclocked configurations, but between different HD 2900 XT GPUs. It seems TSMC's 80nm HS process allowed ATI to clock the R600 relatively high, but once the upper limits of the GPU's clock speed are hit, power consumption can vary considerable from card to card. Having the 8-Pin supplemental feed available means there should always be ample power available to the card.
To find the Radeon HD 2900 XT's peak core and memory frequencies, we slowly raised their respective sliders until we begun to see visual artifacts on-screen while running a game or benchmark, or until our test system was no longer stable.
In the end, we were able to take the Diamond Viper Radeon HD 2900 XT up from its stock GPU core and memory clock speeds of 742MHz / 1000MHz, to 858MHz / 1089MHz. While we had the card overclocked we re-ran a couple of high-resolution benchmarks and saw significant performance increases in both. F.E.A.R. in particular liked the cards higher clocks, which resulted in a 7 FPS increase.