Adding a second HD 4890 in CrossFire raised the performance in 3DMark Vantage but it didn't help at all in Far Cry 2. This came as something of a surprise but we always knew we would have to use Nvidia graphics to investigate PhysX performance so we bit the bullet and switched from HD 4890 to GTX 280. Nvidia's graphics drivers can shift the PhysX workload from the CPU to the GPU. The idea is that this is a more efficient use of the resources inside your PC and if you take this approach to its logical conclusion you can add a graphics card that is dedicated to PhysX.
This PhysX behaviour means that 3DMark Vantage can treat a suitable Nvidia graphics card as a pseudo CPU so installing a GTX 280 results in a leap in CPU performance in the benchmark compared to an HD 4890. Adding a second GTX 280 in SLI raises the GPU performance considerably but has little effect on CPU/PhysX performance. After that we added a third graphics card in the shape of a GeForce 8800 GT and used the graphics drivers to devote it to PhysX duties. This raised CPU performance by a small amount from about 48,000 marks to 50,000 marks or four percent.
These results were clear to see in 3DMark Vantage but were quite invisible in Far Cry 2, although the benefits of SLI were quite evident.
The problem, from the point of view of the E760 Classified, is that the results were identical for both the E758 and the E760 and it seems rather steep to pay an extra £100 for the Classified when you don't get any extra performance in return.
Of course the Classified has a trick up its sleeve as it has four graphics slots and not just three, so you can install a PhysX card in the third slot with another double slot graphics card in the fourth slot hanging off the bottom of the board. Unfortunately it will block the micro buttons and the Firewire header. In theory it would be possible if we had a third GTX 280, but we do not, and we suspect you don't either.
Our view on the subject is that it would be rather pointless to install four graphics cards as the combination of Core i7 and three graphics cards is quite awesome. A fourth card for PhysX is not so much the icing on the cake as a light sprinkling of hundreds and thousands but the cost is very high and you'll need a substantial stack of graphic cards and a beefy power supply to boot.
The EVGA X58 SLI Classified is a thing of beauty but the fourth graphics slot is something of a white elephant and the price is little short of scary.