We'll kick off our testing with power and noise. Notice that the cards marked with asterisks in the results below have custom cooling solutions that may perform differently than the GPU maker's reference solution.
Turn your attention first to our comparison between the pairs of sub-$200 video cards, the 6850 and GTX 460 768MB, versus a single GeForce GTX 580. The GTX 580 consumes a little less power at idle, but when running a game, the 580 pulls more juice than either the 6850 CrossFireX config or the GTX 460 768MB SLI. The multi-GPU solutions don't look too bad on that front, at least.
As for the GTX 580 SLI rig, well, that requires considerably more power than anything else we tested, a hefty 575W at the wall socket under load. That's a lot, but as we'll soon see, such power draw may be appropriate to the performance achieved.
Noise levels and GPU temperatures
Let's focus first on the Asus 6850 and GTX 460 768MB cards with DirectCU coolers. In single-card configs, they achieve among the lowest noise levels and GPU temperatures we measured. They have substantially less power to dissipate in the form of heat than some of the higher-end solutions, but stillthis is an impressive performance from that DirectCU cooler. I prefer the tuning of the 6850 to the GTX 460 768MB; the 6850's GPU temperatures are a little higherthough still lower than mostand it's measurably quieter under load. That difference comes to the fore when we add a second video card next door. The GTX 460 768MB works hard to keep the primary GPU's temperature relatively low, at 73°C, and that shows on the decibel meter, where only the GTX 460 1GB cards are louder. The 6850 CrossFireX pairing, meanwhile, maintains decent temperatures but registers substantially lower noise levels.
Happily, the bottom line here is that Asus' DirectCU cooler is excellent when the adjacent slot is unobstructed and, crucially, still adequate in multi-GPU configurations. With similar thermal loads, though, the 6850's fan control tuning is quite a bit more acoustically friendly.
Meanwhile, the GeForce GTX 580 has a very good cooler of its own, and as a result, it's quieter than both the GTX 460 768MB SLI and 6850 CrossFireX setups. Adding a second GTX 580 results in slightly higher noise levels and GPU temperatures, but heck, the 580 SLI setup isn't much louder than a single Radeon HD 6870.