XfX 6600 GT overheating

Does the card overheat within 20-30 minutes, or is it now acting up instantly in games, indicating possible damage?

You could try underclocking it back to reference 6600GT clock speeds, and see if that helps....

My bad. I'm good now.
My main concern was that it was crashing during some intensive MMPOG activity, WOW and Guild Wars.
I took someone else's advice and rolled back the drivers (Forceware 74.12, if memory serves). That seemed counterintuitive, using year-old drivers, but that solved all my problems.

After a good burn in on my Artic Silver thermal compound, my AMD 64 3000+ Winchester is now averaging 29 C at idel, rarely climbs above 34 C at load (I've got that big all-copper Zalman cooler on it too.)
System temps, mid 30s.
GPU, according to the NVIDIA tuning software, climbs up to about 45 C.
It had been getting up to nearly 60 C, but adding the slot fan really seemed to help -- it pulls the heated air out what had been dead space between the card and the bottom of the case.
Anyway, all problems solved now, thanks for the advice.

Thanks.
The air flow seems right.
My video card seems to be sucking air in through the plastic shroud, across the memory and GPU, and exhausting it out toward the Mad Dog slot fan, which sucks in this hot air, along with air trapped in the bottom of the case, and exhausts it out.
I've noticed that the air streaming out at the bottom of my case through this new fan is noticeably warmer than the air exhausted by my main case fan.
That tells me that my video card was generating a lot of heat that was congregating at the bottom of my case.
Also, my memory was faulty, I'm using the version 71.24 drivers for my Nvidia card (not 74.12), and after several days using these older drivers I've had no more reboots or black screens.
So, a glitch with the drivers turned out to be my main problem, but added cooling to my case also helped.
Thanks to all.