I'm on the fence with 5 series GPUs. I personally think they are too overpriced and only the introduction of Fermi from NVIDIA will reduce the prices to reasonable levels, but in saying that, I still would recommend them to people. It depends, is this computer going to be used to play current generation, or next generation games. i.e. are you one of these people who'll buy the game on the release, or after 2 or 3 years when it's in the bargin bin?
If it's next gen games, I would recommend you get a 5850, if you can actually find one, failing that a 5770. If you grab games form the bargin bucket, you'll be fine with a 4870 or 4890 depending on what your budget allows.
Then there is the old pickle of PhysX, do you need it? If so, consider getting something like a GTX 260.