Tweaking Part 1 - The BIOS
Making sure that your card runs as fast as possible starts in the BIOS. There are several relevant settings here that can influence the performance of your video card. Not enabling some options will mean some software tweaks will have no effect!
AGP Mode - Determines the AGP frequency, as a multiplier of 66MHz. 1X is the slowest, 4X is the fastest. Some motherboards will have an "auto" setting, which determines the proper rate from the card in the AGP slot. Others might have an "AGP 4X Mode" enabled/disabled type setting, enabled will set 4X, disabled will set 2X. Older Intel BX and VIA Apollo motherboards will not have the 4X option as the chipsets do not support it. If the setting is there, set it to the highest setting available, which for most people is 4X.
AGP Fast writes - Determines whether the video card memory will be allowed to be written to directly, instead of going through the system memory. This setting should decrease the time it takes to transfer data to the video card for processing, as it bypasses the memory controller. Enabling it will increase performance, but quite possibly at the expense of system stability. Some motherboards will not have this option, but generally those with overclocking features have it present.
AGP Sideband Addressing - Allows the AGP card to use system memory for texture storage, a feature made famous by Intel's i740/i810. Not really relevant with today's 64Mb and 128Mb cards, due to the additional latencies generated by using the system memory instead of graphics card memory. Often this setting is not available on newer motherboards; if it is there I would suggest disabling it.
AGP Aperture - Should always be equal to or higher than the amount of memory on your video card. A general rule should be to set it at double the graphics card memory.