If you think you might be using Premiere CS5.5 at all, it does have CUDA acceleration. So, a second-hand (or hacked) GTX 285 or a Quadro 4000 would be the best choices there. So for a case like this, order with the stock 5770 and swap it out with an NVIDIA, if you want to take advantage of the CUDA acceleration.
In FCP 7, the graphics card doesn't matter as much, although Apple Color does tend to perform best on ATI GPUs (better optimization and more output options). In this scenario, again, the 5770 would probably be a fine choice. With the exception of some plugins, FCP 7 almost never uses the GPU, so spending a ton of money on a faster one would be pointless. Color and Motion are really the only apps in the suite where that matters and honestly, they both perform very well on the 5770.
In FCP X, GPU acceleration is based on OpenCL. Here, the 5870 graphics card option would be in theory better (twice the processing power). Also not a bad idea if you plan on using the machine for gaming as well.
