How the X2 handles
I am pleased to report that AMD has made a breakthrough in removing one of the long-standing problems with multi-GPU configs: multi-monitor support. Traditionally, with SLI or CrossFire enabled, a PC could only drive a single displaya crazy limitation that seems counter-intuitive on its face. If you had two displays, you'd have to go into the control panel enable CrossFire, thus disabling one monitor, before starting up the game. You'd then have to reverse the process when finished playing. Total PITA.
Happily, though, AMD's newer drivers pretty much blow that limitation away. They've delivered the "seamless" multi-monitor/multi-GPU support AMD promised at the time of the Radeon HD 3870 launch. Here's what I was able to do as a result. I connected a pair of monitorsa 30" LCD with a dual-link DVI port and an analog CRTto a Radeon HD 3870 in a CrossFire pairing and enabled CrossFire. Both displays continued to show the Vista desktop with CrossFire enabled. Then I ran UT3 and played full-screen on the LCD. The CRT continued to show the Vista desktop just fine. Next, I switched UT3 into windowed mode, without exiting the game, and the transition went smoothly, with both displays active. Finally, I dragged the UT3 window to span both desktops, and it continued to render everything perfectly. Here's a screenshot, wildly shrunken, of my desktop session. Its original dimensions were 2048x1536 plus 2560x1600, with the UT3 window at 2560x1600.
That, friends, is what I'm talking about. I tried the same thing on the 3870 X2, and it worked just as well. By contrast, enabling SLI simply caused the secondary display to be disabled immediately.
This multi-display seamlessness does have an unhappy consequence, though. Since multi-monitor configs work seamlessly, AMD seems to think it can get away with essentially "hiding" the X2's CrossFire-based nature. The Catalyst Control Center offers no indication that CrossFire is enabled on an X2 card, and there's no switch to disable CrossFire. That stings. I've worked with multi-GPU setups quite a bit, and I wouldn't want to own one without having the option of turning it off if it caused problems.
The one indication you'll get on the X2 that CrossFire is involved is the extension of the antialiasing slider up to 16X. This 16X setting triggers CrossFire's Super AA mode, a method of multi-GPU load distribution via cooperative AA sampling. This is a nice option to have, but in my book, SuperAA isn't the best option for load sharing or for high-quality antialiasing. I'd much rather use 4X multisampling combined with one of the Radeon HD 3870's custom tent filters.
AMD is running behind on another of its promises from the Radeon HD 3870 launch, unfortunately, and that affects the X2 directly. At the time, the firm said it would be providing drivers for "CrossFire X" in January (that's now) to enable CrossFire with three or four GPUs. However, AMD has delayed those drivers until March as it works to get adequate performance scaling out of more than two GPUs. That's no small challenge, so I wouldn't be surprised to see the schedule for the drivers slip further. Eventually, AMD expects the 3870 X2 to be able to participate in a couple of different CrossFire X configurations: dual X2 cards forming a quad-GPU phalanx, or one X2 card hooking up with one regular 3870 for some three-way action. I'm curious to see how well this will work once the drivers arrive.