Well the good news is you can't see a difference between 120FPS and 80FPS with your monitor anyway.
The way I see it, the thing with your rig is that while the A64's are no speed demons by any means these days, they are still *capable* as I'm sure you game 'ok' on your rig.
At the same time, the 8800GT is not a bad GPU by any means, but it's certainly not going to max games out at your resolution either.
You're in an spot where you're likely to see gains from an upgrade of either component in different areas/games. A better upgrade path for you might be to skip both the 4870 and PhII 940 and look at upgrading to a PhII 720 (it'll provide 99% of the performance clock for clock in almost every game I would think compared to the 940... I'm sure someone will dig up a bench to prove me wrong though. ) and then look into a logical GPU upgrade with the money you save plus the money you get for selling your old components, and maybe a little more saving. The 4890 is about to be released, so is the GTX275. Both are expected in April. I wouldn't be suprised if GTX260 and 4870 prices drop a few bucks more... already you can find them both for very near $150 if you look around and are patient. I bet by April $150 for these parts will be more common.
I just wouldn't upgrade to a GTS250 or 4850 type of card with your PhII 720 (if you go that route), they'd both be faster, but in my opinion they wouldn't be so much faster that I'd spend money on them over an 8800GT. That's why I'm suggesting a PhII 720 now (and save a few bucks over the 940) and then do a proper upgrade in the GPU area. Just my $.02