Several things came to mind when I read through the replies here.
First, a while back I had problems with a certain set of video drivers for my 6600GT card. I didn't test it on all games, but at the time EQ1 would lock up after a random amount of time. I restored to earlier drivers and all was fine again. I believe the problem drivers were in the early 7x.xx series (I think 71.84), and the one I went back to that worked was 66.93 or so. Since then, I've switched to 77.72 as I've read a lot in forums saying they were very stable on both older games and newer ones. I've also seen that it does improve in several of my games, including EQ2 and the annoyingly choppy Qeynos Harbor area. Just make sure to clean the system of the old drivers before you update, and turn off antivirus programs when installing the drivers. Look for programs like Detonator Destroyer on
Guru3D that can completely wipe any leftover driver files off, so you can avoid version conflicts.
Second, if you suspect that the system might be overheating, you can get programs to monitor the heat. One I use these days is
SpeedFan. Another you can try is
Motherboard Monitor, but it's a lot more complex. SpeedFan operates in the system tray, so you'd have to play games in a windowed mode to see it. MBM can do an overlay display within games. Chances are though, given you have a 6600, you can check the temp of the card itself in the Display Properties --> Settings tab --> Advanced button --> GeForce 6600 tab --> Temperature settings in the left menu. Just be careful of changing anything in there unless you know what you're doing.
Third, if it is a problem of heat, you can take steps to ventilate the system. They all depend on your situation and money. The simplest is to just open up the side of your case and point a small fan to blow into the case. But if you have any pets, they might get curious as to what's inside there. If you can afford it, get some 60mm (size depends on the mountings/vent holes in your case) case fans and install them. Just be sure to remember to balance out the intake and exhaust. Too much intake, and your case becomes a dust bunny repository. Too much exhaust, and you can't draw in cooler air to replace the hot air inside. Those case fans are usually pretty cheap if you shop around. Somewhat more expensive would be to buy or replace heatsink/fans for the components: VGA chip, CPU, northbridge, and memory (if you're overclocking).
All this depends on how comfortable you are in fiddling around with your computer. You could also try another video card if you can get your hands on one long enough to test that. If nothing else works, back up what data you can and format the drive and reinstall Windows and install the latest drivers for everything, in the hopes of avoiding too much system clutter and conflicting driver data. But that's last ditch, if you're totally fed up.