In all honesty, Iv been looking in to the whole 5850 vs 5870 vs gtx 470, recently as I have been thinking of upgrading my 3870x2.
Although benchmarks are user opinions vary, over all, it seems the gtx 470 is no better performance wise (raw performance apples to apples) to a 5850.
Its only benefits from the close developer association Nvidia has with game developers.
However...... becuase fermi is 6 months late, lots of game developers actually had to take a look at ATI hardware too as it was the only dx11 hard ware available.
Saying that the 5850 is a very good card.... close enough to a 5870. For the price being £100 cheaper is a goot price performance card.
Myself..... I know I would hate my self for not spending that little bit extra and not getting a 5870, so if I was to get a card a 5870 it would be.
The 5870 is close to a GTX 480 and is £100 cheaper.
The ONLY advantage Nvidia have over ATI is tesselation.
But what you have to consider is, the 5000 series cards are for todays games..... where as nvidia cards are better prepared for tomorrows (games to come in 1-2 years)
Saying that thats why ATI cards are better price/performance cards to have now than the nvidia equvalents.
Im sure the next family of ATI cards 6000 series, will have better tesselation and dx11 technology.
It;s just not that important yet, becuase, there really isnt that much dx11 titles out, or the api isnt even that much developed.