If it was my money and my upgrade criteria was when I'm unable to play the latest games at all, I would get a 7850 instead of a 7950, and upgrade sooner. That way I would always get good performance at medium to high settings, rather than really good performance for a few years, but worse performance for some time before I upgraded.
If you don't need the newest games on max settings, with high fps, then I think its better to spend less now but upgrade sooner.
Anyway that's just my opinion, its your money at the end of the day.