GTX400 series have yet to available in any appreciable quantity and as such have not settled at their market price yet. It is the way of high end GPUs that they start out at their MSRP but they then sell out and the price often jumps about $100. The same exact problem happened with the 5850/5870/5970 initially.
A 20% performance increase is optimistic at best and painfully ambiguous at worst. I'd look at it on a case by case basic as the result can vary quite widely between different resolutions, AA levels, etc. The impression I got from reading [H]ardOCP's review is that the GTX470 isn't worth the price premium over the 5850 as it offers little to no performance gain but comes at significantly greater cost (assuming it keeps selling at MSRP). Similarly the GTX 480 competes with the 5870 and out performs it in a few games, but certainly not by a margin that justifies the additional cost. The one scenario that Nvidia's latest GPUs did come out on top, [H] mentioned, is in GTX480 SLI which has really impressive scaling compared to that of the HD5970. This is, of course, the extreme high-end and will be quite expensive. The power requirements are enormous for such a system.
http://www.hardocp.com/article/2010/03/26/nvidia_fermi_gtx_470_480_sli_review/8If you were to buy a GPU in the next few months I would still go with ATI as it will be quite awhile before we see the GTX400's regularly available. The HD5830/5850 are quite good for mid-range systems and the 5870/5870CF/5970 make great high end cards. The HD5770 is a well priced, good performing, budget GPU.
