Crysis
I was a little dubious about the GPU benchmark Crytek supplies with Crysis after our experiences with it when testing three-way SLI. The scripted benchmark does a flyover that covers a lot of ground quickly and appears to stream in lots of data in a short period, possibly making it I/O bound—so I decided to see what I could learn by testing with FRAPS instead. I chose to test in the "Recovery" level, early in the game, using our standard FRAPS testing procedure (five sessions of 60 seconds each). The area where I tested included some forest, a village, a roadside, and some water—a good mix of the game's usual environments.

Due to the fact that FRAPS testing is a time-intensive endeavor, I've tested the lower-end graphics cards at 1680x1050 and the higher end cards at 1920x1200, with CrossFire X included in each group.

This is one of those applications where CrossFire X can only make use of three GPUs due to limits on how many frames the driver can render ahead. As a result, four-way CrossFire X performs the same or apparently slightly slower in Crysis. Of course, since we're playing through the game manually, some variance in the scores is likely. I'd say CrossFire X four-way performs essentially the same as three-way.

Also, CrossFire X is of no benefit in Crysis at 1680x1050 resolution with these quality settings. At 1920x1200, adding a third Radeon HD 3870 GPU does raise average frame rates slightly, but the median low frame rate doesn't budge. My seat-of-the-pants impression is similar: the game doesn't play any better with a third GPU.

In order to better tease out the differences between two, three, and four GPUs, I cranked up Crysis to its "very high" quality settings and turned on 4X antialiasing.

None of the graphics solutions produce truly playable performance, but we do see a clear difference between two and three Radeon HD 3870s. Note that, although CrossFire X manages higher average frame rates than two 8800 Ultras, its frame rate minimums are lower. The reality here is that, for practical purposes, having more than two GPUs is no help in Crysis right now.

Unreal Tournament 3
We tested UT3 by playing a deathmatch against some bots and recording frame rates during 60-second gameplay sessions using FRAPS. This method has the advantage of duplicating real gameplay, but it comes at the expense of precise repeatability. We believe five sample sessions are sufficient to get reasonably consistent and trustworthy results. In addition to average frame rates, we've included the low frames rates, because those tend to reflect the user experience in performance-critical situations. In order to diminish the effect of outliers, we've reported the median of the five low frame rates we encountered.

Because UT3 doesn't natively support multisampled antialiasing, we tested without AA. Instead, we just cranked up the resolution to 2560x1600 and turned up the game's quality sliders to the max. I also disabled the game's frame rate cap before testing.

Here's another case where CrossFire X scales up to three GPUs better than Nvidia does, and this time, it's enough to put the Radeons over the top. Adding a fourth GPU is no help, and it even seems to hurt performance. In fact, this may be one case where rendering too far ahead causes problems. Four-way CrossFire X didn't seem to play UT3 particularly smoothly, and I had trouble (more than usual) with placing shock rifle shots, too. Whatever the cause, I was able to be more accurate with three or fewer GPUs.