waiting will always reap better rewards, but unfortunately no matter how long you wait, something else will be in the pipeline. if you're running a system that can run current games to a standard you desire, then don't bother with the next gen cards
but by all means don't wait a year for something when you could have great performance for that year, and still great performance for a while after that too.
There always has to be a balance in how long you're willing to wait, and what you'll miss out on if you do wait. A year is just a little long for a gpu, especially when the gpu in question is just a rumour, and the timescale likely to be a lot longer than a year. After all, a hd5870 will still run new games for a while and very well too (games can't be much more demanding than crysis really for the next 2 years, and that runs nicely
). Also, the next generation of consoles won't be out for a year or two anyway, so the majority of AAA games on the pc will run on any midrange card until then. Well, probably a few months after that, the first real 'next gen' game on the 360 was GoW i think, all the rest were just ps2 ports for about a year. Considering the disparity between the 360 and ps3 release dates, i see a repeat of that situation [although the ps3 does have a bit more power anyway, so it won't be as bad].