I went from a 7950GT to an 8800GTS 512 a long time ago. I wondered the same at the time, whether it was worth it or not. Honestly, the real world difference is more than what you see in the benches. It's not just a matter of clear-cut FPS. Shaders helped a ton, AA seemed like it cause WAY less of an FPS hit, and the gaming experience on the same games vs. the 7950 was just overall a lot more enjoyable for me.
Might not make sense to you, but it's how I saw it when I made a similar upgrade.
If you can get the 8800 cheap, definitely go for it.
I also did that upgrade while playing WoW. I can tell you that is one game you will see virtually no improvement in with the VGA upgrade.