This is a quick and mini guide on OpenGL that I’ve used to help me out with games and might be of use to others who have problems or want an increase in performance.
Benchmarking
First off, know your system. Using a syntetic benchmark may not be an actual performance measure of game playability, but it does help to inform you of the limits of your current setup and how changes affect it. If you don’t like this idea, create your own demos inside games like QuakeIII so you can play them back with different settings.
Also if you want to see your frames per second try something like http://www.fraps.com/ , they have a free version that runs great and doesn’t eat up very much resources. Fraps is a background program that will read your FPS pretty accurately and will also allow you to record gameplay movies and take screenshots.
Monitoring
Also, maybe install a hardware monitor that is compatible with your motherboard to read the temperature of your CPU and inside your case–some graphics cards have this option as well. Know their limits and realize, the hotter it gets in your case, the hotter everything gets in your case. Getting random lock ups? Computer resetting for no reason sometimes? A good number of these errors are caused by a system or one of its componets overheating.
Over Clocking
It’s all the rage with game nuts. Basically, you better be running a hardware utility program for every piece of hardware your going to overclock. Read up on O’clocking website before you try it, and know what your doing. Don’t jump to higher speeds, increase your mhz in as many small steps as possible and run your system for a good while keeping an eye on temperatures. If your seeing artifacts, or pixels of a game that are not updating correctly, ie little blocks that are not changing colors with everything else - your running your graphics card too hot. If you see lots of lines as well with the same properties, it’s a good sign of too much heat. Take it back down a notch and realize that to o’clock you need good hardware and good cooling, which in the end - usually costs more.
Drivers
First off, remember drivers are drivers. They are made to “drive” sales of a company’s graphics cards. If a certain game is very popular currently, then you can bet the next set of drivers from a company will be aimed at that game particularly and all other games will either benefit, or not. What am I saying? If it isn’t broke, dont fix it. 
If your runnings games that are a little older or not as popular as the latest kill everything that moves FPS, new drivers are not necessarily always the best move. Now dont get me wrong, new drivers usually are a better move, but I’ve seen games drop in performance just because new drivers are not aimed at improving some older or less popular games.
Now, when you install new drivers for a graphics card, Uninstall your old ones first! This, again, usually isn’t a problem, but when it does become one-you’ll know about it. Yes I know its an extra reboot in XP and thats such a drag, but being extra carefull never hurt anyone.
AGP Aperture Size
What is this? Its a setting in your bios that dertermines how much RAM your system will share with your video card. “So I should turn this up as high as possible, right?” Umm no. When you take away RAM from the system, other processes might suffer. Games aren’t just graphics From my experience with OpenGL games and some DX games, 16mb should be the absoulte lowest value it ever goes too, and nowadays that shouldn’t even be that often. Now this does depend on your system RAM as well, but with half a gig of RAM I seem to hit a sweetspot with most games at 128mb AGP Aperture size. Basically the more CPU intensive a game, the more RAM your going to want to leave for the system to use. I’ve seen some games decrease at 256mb and wouldn’t recommend trying this value unless you carry a gig of ram in your box.
In the end every box is different and most people don’t buy quality parts or the best video cards. So when you read reviews of how this card did this much better than that card, remember that the guy doing the review has the latest and greatest system and unless your paying top dollar for your system, dont expect the same results. If your paying less than $200 for a new graphics card, don’t expect to be turning on AA and AF very high or at all to keep a game looking good and running smoothly.
Well there you have it, not sure why I took the time to talk about this stuff, but hopefully someone will take something away from this. And don’t forget, forums are your best friends! Especially the search functions! This forum is pretty packed with common answers to questions and try other places like http://www.tweak3d.net/ or http://www.nvnews.net/ for Nvidia cards info.