Mr Frag is indeed correct...your CPU will indeed struggle to use the 6600GT to it's full potential...actually...it won't be able to keep up.
Replacing 1 component will sometimes work...but not always...sometimes it's just good to start fresh...however, if you have no foreseeable money coming that would enable you to start again, then I'm sure you would see the benefits fo a 6600GT. You certainly wouldn't be able to push it...but graphical, things should be on the up with a 2.2GHz.
Again, though, if you go and put a 6600GT AGP card in your computer and, in the enar future, upgrade to a new syste, that's the AGP gone because, as Mr Frag rightly said (again), components seldom cross over...and new motherboards are using PCI-E.
I have a 3200GHz AMD 64bit with 2GB ram and a 6600GT and my system has problems running some games (X3-Reunion) on maxed graphics...however, others are no problem. GT Legends >=60FPS, Quake4 >30FPS (except some places) and CoD2 >= 60FPS. Somtimes that will be down to code, though - how well the graphics engine is coded.
In short, if you see money coming your way and can wait, then wait...otherwise, I'd go ahead and stick the 6600GT in...I'm sure you'll see a difference. And it is only £100!
Alba gu' brath