A fresh Multi-GPU Performance Hotfix has become available for Windows Vista users which deals with a DMA latency issue of sorts between the two cards. Microsoft describe it in full detail below :-

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.

If you're a serious gamer running a multi-GPU setup in Vista, this is a must have!