Fastest high-end: X800XT/PE, as I believe it wins most of the benchmarks at high res, AA+AF.
Fastest $280-320: This is a weird, hyper-specific price range. Given the next two questions, I would've expected $250-350 as the choice. I think the semi-regular $330 X800P is very tempting compared to a ~$275 6800, given the former's 256MB memory and much higher core speeds. But it's a relative toss-up. The 6800 is great for well under $300, and a X800P is tempting for well under $350. Obviously a 6800GT is preferable over a X800P if their prices are at all comparable. No vote for me b/c it's too specific a range (the 6800 is the only card in this price range). Barring the odd sale, a 6800 wins at $200-300, and a 6800GT wins at $300-400. Without a decent game bundle, though, I'm not sure how good a $250+ 6800 looks compared to a $200 6600GT. No vote from me.
Fastest $150-250: Way too big a spread here. This ranges from a used 9800P to a new 6600GT to a new 6800. I voted nV, as it gets the nod for new cards in this range (and basically owns the AGP market at this price range).
Fastest $50-150: ATi gets the nod for its used 9700s, 9700Ps, 9800s, and 9800Ps at $100-150. Below that, you're getting kind of slow for newer games. I'm upgrading from a 9100, and I'm not considering anything less than a 9700P (which is slow enough in some newer titles, compared to a 6600GT/X700XT).