They're already close to $350 with rebates. They will be gone by the Fall most likely.
They leave a big gap to fill between the 8800GTS 320MB and 8800GTX, that will probably be filled by some kind of 8850GTS or 8900GTO or something. A GTO card would basically a cut down 8800GTX with less stream processors and only 512MB of DDR3, where an 8850GTS would be a suped-up 8800GTS to end up at a similar specification. Meeting in the middle basically, either by cutting down the GTX or adding to the GTS.
They need a card beneath the 8800GTX that has 512MB or more of RAM, because 512MB was becoming the norm for high end on the last gen cards and ATi's cards will have 512MB, leaving 320MB GTS falling short on the high res AA/AF benchmarks where lots of texture data needs lots of RAM. Most likely we'll get some sort of 8850/8900 that basically replaces the 640MB 8800GTS with a 512MB card with similar stream processor and bus-width numbers as the GTS for $399 MSRP to compete directly with the 2900XT. The 8800GTX will compete with the 2900XTX (which we have not seen true benchmarks for yet, contrary to DT confusion) and the 8800 Ultra will be the champ for single-card benchmarks. Just my guess.
Probably some 8800GS comes out that has the performance the 8600GTS should have had (64 stream procs, 256-bit bus). That should take care of competing with anything ATi releases beneath the 2900XT.
Basic order then:
8800 Ultra
8800 GTX / 2900XTX
8850/8900 GT_ 512MB / 2900 XT
8800 GTS 320MB
8800 GS 256MB?/ whatever ATi card is beneath the 2900 XT, probably w/ 256MB
poo-poo 8600 GTS