Although I havn't seen any benchmarks for NV30(those that have would have to kill you if you see them) I saw some marketterese(can't... remember... where... possibly in japanese?) somewhere that showed a graph with a ti4600, a 9700 positioned at about 20% faster than the ti4600, and then nv30 positioned about 250% faster than ti4600. It is a completely new chip design, unlike anything since the orig GF3, or the orig gf256. In addition, it is a die shrink(this appears to be why its taking them so long). This makes the NV30 performance a wildcard.
If my barely comprehensible previous paragraph hasn't convinced you, I say this: At least wait until Nvidia puts out engineering samples, and the inevitable stream of leaks and benchmarks that follow, to buy a 9700. Its still likely to be a good value even then, but if it isn't(If, for example, Nvidia puts out a 4200-equivelent NV30 thats easily overclocked and starts out cheaper than the 9700) then you won't be regretting it. To get in on the Xmas market they have to be taping out at least around now(I give 6 weeks from tapeout to sampling, 6 weeks from sampling to limited retail). In addition, by then you will be able to see some stats for Hammer, and see if you want to save your cash for it.