The Radeon 5870 is the fastest single processor graphics card on the planet, ( people will dissagree, but it is ) so a 5870 is the right choice. id reccommend at least a 750 watt PSU, or higher if you plan on getting another 5870 down the road.
This is wrong, plain and simple. Best Price/Performance, Power Consumption and Heat may be an issue for some and it may not be an issue for others. The Fastest is the Fastest regardless of how much it costs or how much power it uses. The GTX 480 is the Winner now and it will keep getting a little faster for the next few driver releases.
With that said I'm sure ATI has their Evil little eyes working on something new or improved.