The best money can buy would be the GeForce 6800 Ultra Extreme Edition which costs about 600 bucks last time I checked it out. My 6800GT OC from BFG only set me back 430 bucks(with tax) and I easily over clocked it to 445/1200 but run it everyday at 430/1200.
But, with the X800 XT PE only a few frames per second behind, at times ahead, its also a kick-ass card so its really a brand preference. Unless you want the support of SM 3.0, then go with nvidia.
As for a value card, 9800Pro 128mb no doubt.