It's useful for more than just backup. 8800 has 112 cores against the 285's 240, so might well provide useful further acceleration for any OpenCL applications. My Mac Pro CUDA rig uses both 8800 and 285 GPUs together under bootcamp and does the multiGPU examples nicely, so OpenCL should manage the same under 10.6. 285 is about twice as fast as 8800 under CUDA, so you might get 50% boost under OpenCL keeping it in your Mac.