I've done a bunch of research on it as well, and in my opinion, the list should be:
4870 x2 > GTX 280 > HD 4870 > GTX 260 > HD 4850 > 9800 GTX+
And if you wanted to put it in, the 9800 GX2 would be the one just above the 4870.
This is a good list: Graphics card Rankings
But it's hard to compare between ATi and Nvidia. It depends on the game in most cases, and there's never a huge difference. It also might be more worth getting an nvidia if you picked out a motherboard with SLi, for example. Besides, both cards can easily play any game on the market, and both will be usable for years, so there should be no regrets in purchasing either. It also depends on resolution. I play on 1280 x 1024, so in my case, there wouldn't be much of a difference between any of the cards mentioned. I'm only going with the 4850, because the higher cards would honestly just be overkill and I'd feel like I wasted money.
And for benchmarking, companies should compare the cards with both resolution, and anti-alias, not just on one resolution with one set of settings on 4 randomly selected games.