We've now had a look across the four SLI video card combinations that are available, and yesterday we made some telling conclusions about the value of jumping right in and purchasing an SLI setup without thinking. SLI is an exciting architecture, but you do need to spend some time investigating what is the right solution for you.
Hopefully this article will help to clear up some more myths of SLI. We are going to be evaluating the real-world gaming experience that two GeForce 6800 GT's can deliver when we scale the clock speed of our Athlon 64 FX-55. This should give you a ball park of how two GeForce 6800 GT's perform with a slower CPU, and answer the question of just how far SLI is CPU limited.
For this evaluation, we have use three different clock speeds for our Athlon 64 FX-55. To represent the high-end, we have used the FX-55 at its default 2.6GHz clock speed, while we clocked it down to 2.2GHz and 1.8GHz to represent the performance mainstream and mainstream processor ratings that AMD use.
With our FX-55 at 1.8GHz, we feel that the performance should fall at around a 3200+, which is clocked at 2.0GHz or a 3000+, clocked at 1.8GHz, due to the cache differences. At 2.2GHz, we know that the performance of our 2.2GHz Clawhammer-based FX-55 will be in the same sort of ballpark as a 3700+ San Diego.
How We Tested:
Please be aware that the way we test video cards is not a like-for-like comparison, and it is not meant to be. We decided to concentrate on finding the “Best Playable” settings - this means that we're finding the best possible gaming experience delivered on each different configuration. There are no time demos used in our evaluations - we're focusing on the real-world gaming experience, which is, ultimately what should determine your next graphics card's purchase.
AMD Athlon 64 FX-55 (operating at various clock speeds - see below); 2 x XFX GeForce 6800 GT (operating at their default clock speeds of 350/1000MHz in SLI mode); DFI LANPARTY nF4 SLI-DR (NVIDIA NForce4 SLI); 2 x 512MB OCZ Enhanced Bandwidth PC3500 (operating in dual channel with 2.5-2-2-5 timings); Western Digital 200GB Caviar SATA 150 Hard disk drive; OCZ PowerStream 520W Power Supply; Windows XP Professional Service Pack 2; DirectX 9.0c; NVIDIA NForce4 Standalone chipset drivers, version 6.53; NVIDIA Forceware display drivers, version 71.89.
Processor Speeds (PR Values):
In all cases, we have used an Athlon 64 FX-55, but we varied the clock speed to represent different price points in the CPU market.
1800MHz (~3100+) - 9.0 x 200MHz
2200MHz (3700+) - 11.0 x 200MHz
2600MHz (FX-55) - 13.0 x 200MHz
The video card drivers were left at their default settings with the exception of Vsync, which was disabled in all cases.