Most of the newer motherboard have an option in the BIOS which allows you to increase and decrease the AGP apareture size. What this option does is lets the graphics card borrow some of your RAM when it runs out of its video memory. ususally the optimum level is half your RAM size but 128Mb is ususally sufficient.
On some older video cards, this may give a large boost.
If your computer freezes or crashes, then reset this setting to default. Some video cards have problems with too much memory.