They are a waste of money. If you want to upgrade your older cards, and like me prefer Nvidia over ATI, go for a 9000 series card. The performance differences are TREMENDOUS.
So if you have the choice between a 55 euro 8600 or a 110 euro 9600, the latter will be a much better choice. It will run circles around the 8600, no joke.
(this has to do with the memory controller addressing, which should be 256 bits, not 128-bit like in the 8000 series (with the occasional 192-bit).
So even a 7900 with 256 bits will outperform a 8600, even though the GPU is of a later date.
If you do games, or have applications that use OpenGL rendering (videoprograms!) you want to really take notice of this fact, or you will probably wonder why your new card isn't as much of an improvement, as you had hoped.