I think the Nvidia 300 series will have minimal effect on the ATI price. The dye is going to be enormous, it's going to be hot and it's going to be power hungry. It's going to cost a ludicrous amount both to manufacture and for us to buy (you'll probably be looking at £500 for the top single-GPU card). It should be faster than the HD 5870, but then it's 6 or 7 months behind the game, so it ought to be. It's going to be a very niche product. Unless Nvidia make a loss on the cards under the top one, then it's not going to have a massive impact on the ATI prices.
Retailers have artificially inflated the price of the HD 5000 series due to the previously low stock and lack of competition, but unless Nvidia pull a value performance rabbit out of the hat, I can't only see the HD 5000s coming back to their normal price (so about 15% lower than the RRP).
Of course, if Nvidia follow the path they've used for the past few years, all we'll get for the sub £150 market is the renaming of previous cards... they've already started filling the low end of the 300 series with rebrands. There is talk that for the performance cards, the 300 series name will be skipped and they move straight to the 400 series - presumably because of the bad press they've been getting with the rebrands.