When we got our hands on a Geforce 6800GS, we tried to run it with a 6800GT in SLI and it failed. Today we got a second 6800GS from Leadtek and wanted to give it a try in SLI.
We plugged the XFX Geforce 6800GS in the first PCIe graphic slot and the Leadtek 6800GS in the second PCIe slot. We checked the frequencies and learned that both cards are working at XFX default speed. They were running at 485 MHz core and 1100 MHz memory. We checked on the Leadtek specification and learned that its 6800GS is clocked to 425 MHz core and 1000 MHz memory, Nvidia reference clock speeds. The machine with an XFX 6800GS card and the now overclocked Leadtek was very stabile during our first test.
We tested 3Dmark03 and got 22.322 marks and got an idea to switch the cards. We plugged Leadtek 6800 GS into the first PCIe slot and we plugged the XFX 6800GS in the second slot. What happened was no surprise. In this scenario, the Leadtek downclocked the XFX card to 425 MHz core and 1000 MHz. They scored 20.214, some 2.000 less than in the first test.
It's very easy to conclude that the first card is dictating the clock speed of the other. Cards will run together but, if you plug a slower card in the first slot, you will lose some performance. If you plug the faster card into the first slot, you will gain some performance. So we strongly suggest you do the latter. If you buy two cards from the same vendor you don't need to worry about it.
Those two 6800 GS cards together can, in some scenarios, actually be faster than a single 7800 GTX 512 card.
Izvor:
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