Microsoft has released a new hotfix for Windows Vista which should improve performance of CrossFire and SLI systems:
In some scenarios, games and benchmarks perform more poorly than expected on a Windows Vista-based computer. This issue occurs when the games and the benchmarks use multiple display adapters in a Linked Display Adapter (LDA) configuration.
A graphics processing unit (GPU) or part of a GPU link may incur a small scheduling latency on a new Direct Memory Access (DMA) packet. This is true if the DMA packet is submitted to the GPU that has previously run out of work and if another GPU in the linked configuration is very busy. In some scenarios, significant starvation of one or more GPUs in the linked configuration reduces the expected performance of an application.
A supported hotfix is now available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next service pack that contains this hotfix.
To get this patch you need to contact Microsoft, more info over here.