There's always something bigger and better in the works. If you keep waiting for something better, you'll always be waiting. They guy with 8GB of RAM, a Core 2 Quad X6800, dual 8800GTX, a 5TB RAID 5 array, and a 1000W power supply is out of date. Intel, AMD, nVidia, Seagate, etc all have something they're working on that's better, faster, more efficient than what you can buy right now.
There will be a successor for the 8800GTX. The 8800GTX is the successor to the 7900GTX, which is the successor to the 7800GTX, which is the successor to the 6800 Ultra...
The 8800GTX is the fastest single-GPU card you can buy right now. While AMD's DX10 Radeon is slated to hit about the same time as Vista, we don't know how it will perform. That could be worth waiting for to see what performance it brings. nVidia and AMD are for-profit businesses, so, of course they're going to come out with a successor for the GeForce or Radeon families.
Is it worth it? Well, until we see Vista and DX10, it's very hard to say for 100% certain simply because we HAVE NO DX10 GAMES to test it on. Given that it goes far beyond the requirements to be DX10 compliant, and that it's the 8800 family is the only available DX10 cards right now, it's not a fair question. We do know that it wipes the floor with any other available graphics solution.
The gaming expectations you have along with the money you're willing to spend is going to determine if it's best for you. If you can afford the best available and want the best, then you should go for it.