Before we even enter Windows, a few small tweaks can be carried out to improve your graphical performance. Even though they won't give the performance increases that overclocking will give, they are worthwhile and very easy to carry out.
AGP Aperture Size
This number, measured in MB, represents how much of your system memory is available to the video card. This allows the video card to use data in the system memory instead of its own memory, and it allows textures to be stored in the system memory when they cannot fit in the video cards own memory. Obviously you need to set this effectively, or you will lose graphics performance, or even general system performance. Try one of the following settings, and then set it on whichever gives you the best performance.
If you have 256MB of RAM or less, set the AGP aperture size to either 1/4 or 1/2 of your system memory, i.e. 256MB computers should set it at 64MB or 128MB. If you have more than 256MB of RAM, you should try and set it at 1/3 or 1/4.
The PCI bus speed is 33MHz which allows for a maximum data throughput of 133MB/s. While that seems like an impressive number, it actually doesn't give anywhere near the bandwidth needed for applications like 3D games. The AGP bus went some way to change this. The original AGP (1x) had a 66MHz bus speed, which gave maximum bandwidth of 266MB/s. This was then updated to AGP 2x, which gave 533MB/s, and AGP 4x, which gives 1GB/s. The faster bus your video card can handle the better, as more data can be moved in the time period. All new video cards are AGP 4x, but older cards are 2x and even 1x. Set your card to its maximum setting. You should have no stability issues, and you will allow the video card its maximum bandwidth possible. Owners of GeForce 256 card may find they have problems running AGP 2x on VIA chipsets and are thus forced to run it at 1x. If you look around, you can find programs to overcome this problem.
When you increase this, you are increasing the voltage that is being sent to the core of your video card. The purpose of this is to try and get the core to run faster than possible when at a voltage setting. The main drawback to raising the voltage is the increased heat output from the core. If your cooling setup cannot handle the increased heat, the card will do anything from producing artifacts, crashing, or not booting up at all, or a combination of the first two. If you can raise the voltage without heat affecting the performance, you can bring very good increases in overclocking potential to the card.
Note: This does the same thing as raising the CPU/DIMM voltage, but obviously effects a different device.
Raising the Core Voltage can also result in your hardware having a shorter lifespan and a significantly shorter warranty, so be careful.