Apart from buying a graphics card whose bus runs at AGP 8x (8 times the speed of the original Advanced Graphics Protocol standard) there is nothing that you can do.
If your card - tell us what model it is and we'll find out for you - is capable of it but is not being run at this setting, then it needs to be adjusted in BIOS.
If you have a CRT monitor, do this when it's warmed up so that you can see what comes up: press Delete right after the PC is switched on, when prompted to do so at the bottom of the very first screen.
Once in BIOS head to Advanced Chipset Features and find AGP Data Transfer Mode. What is it set at?