wait for a bit, I've heard rumor of some price drops in November, at least on the nVidia side, and there is an almost guranteed price drop after Thanksgiving from both nVidia and AMD/ATI (christmas season, and yes things like graphics cards get dropped for it, either new releases, or big sales, anything to bring in buyers).
G92 has quite a bit of stigma around it, so does R700, generally my view on the hype for unreleased products: take all of the speculated performance, divide by roughly half, and you have a more accureate rough prediction of where the card will sit.
Speaking on past experience (ignoring the FX 5950 Ultra to 6800 Ultra jump) you can expect about 40 to 60% performance increase between top tier generations (so GeForce 8800 Ultra to whatever the top G92 SKU is, the reason I excluded 5950 to 6800 is that was closer to a 90% jump in some applications, not because of 6800 being so revolutionary, but because of 5950 (and its contemporaries) being so (comparatively) slow)
So if we figure that 8800 Ultra is already more power than anyone should need for any game on the market (unless you're playing 2560x1600, in which case it isn't enough power, but 2x8800GTS 640's is) add another ~45% to that performance, and it should be where G92 will sit (this ignores any new features it may bring, like HDCP upgrades, video decoding, various hardware acceleration through CUDA, etc)
Also figure it'll probably cost $699+ on launch, due to hype and so on, and no software on the market will likely need the power it offers (usually takes 2-4 months for developers to catch up).
Add to this the knowledge that most developers are shying away from PCs due to the sublime performance offered by PlayStation 3, Xbox360, and Wii (this should die down in 18 to 24 months, unless Sony and Microsoft decide to release new consoles every 2 years), the other reason they shy away is that the average PC simply can't offer the graphicsp rocessing performance (the performance delta, GPU wise, is too large currently, even though CPU wise its almost nonexistant)
This roughly means that if staying 100% up to date with games is your primary goal, going with a PlayStation 3 or Xbox360 might be a financially wiser choice, however if you intend to build a nice, top shelf PC, going with Radeon HD 2900 or GeForce 8800 would be perfectly acceptable, just ensure you buy a system with some upgradability to it (so LGA 775 would be a good choice, as would AM2, while things like Socket 939, PATA-133, DDR1, etc would be poor choices).
The advantage of this upgradability (especially if you get a high end power supply that can dish out 700 to 900W, such as the Enermax Galaxy 850W) is that you can buy whatever new graphics board in H1 2008 without much problem (now that PCI Express is stabilized as the accepted standard), so you shouldn't be left out in the cold by that.
Although, another general rule of thumb I've learned from experience:
A PC's performance decays at the rate of around $100/yr for graphics cards, or $1000/yr for the entire system, up to around $350 for the graphics, and around $2500-$2750 for the entire system
GeForce 6800GT 256MB, ~$300 in 2004, as of late 2007, you would just now be feeling its age and just now noticing its sluggishness in only a few titles, but its just about enough to make you want to upgrade, or roughly a $100/yr investment, although buying the 6800 Ultra 512MB for $700~, wouldn't give you performance out to 2011, only out to around 2007 as well.
This is viewable in any graphics card purchase, Radeon 9800Pro was around $200-$250 when new in 2003, and lasted to mid/late 2005 with ease, and into 2006 with some strain
GeForce FX 5900XT was around $170 in late 2003, and barely made it into late 2005
GeForce 7900GT was around $250, in 2006, and still provides acceptable performance levels in late 2007, 7800GTX was around $380-$420, and still provides acceptable performance levels in late 2007 (even if its rare to find a working 7800GTX)
Hope this helps a little with deciding when to upgrade