Last few patches= Plumeiting framerate
The 6600 is getting a little long in the tooth as far as graphics cards go.
That seems to be the weak link here.
The Pentium 4 isn't going to help much either. Post a HijackThis and DirectX Diagnostic report so we can take a look. EDIT: You are defragging regularly, correct?
I have a 6750 processor and a GTX260 and I've been encountering the same issue. Prior to the 5/5/09 patch i wasn't getting much graphic slowdown at all, but now it's like i'm running on my old 6800. Things are lagging really bad on large teams with lots of effects. Prior to this patch the GTX260 was able to handle anything this game could throw at it....
It seems with each patch that has been released since I14 has went live my gaming experince has slowly degraded. First day the Magic booster pack went live I crashed none stop.
Now my framerate in any zone other then a small one room is is neting me between 9 to 2 fps since Yesterday's patch.
It can't be my system... I'm on a Dell Dimensions 4700 with 2.5 Gigs of ram, Pentium 4 HT Processor, AND a Nvidia 6600 Graphics card with 512mb of ram on card with the latest graphics drivers! >_<