ATI Rage Pro on Windows XP

I am using an ATI Rage Pro on a Windows XP Service Pack 2 system. With
the latest ATI drivers, hardware 3D rendering will not go above the
resolution of 800x600 at 16 bit color. My system is a Dell Optiplex
GX1 with a PII 400mHz processor and 512MB of RAM.

I figured out the solution myself. If you have 4MB of graphics RAM as
my system does, you cannot render at resolutions higher than 800x600.
With the 8mb memory upgrade, however, you can reach higher resolutions.
Does anyone know where to obtain an upgrade?