More RAM in Linux

I can't get Linux (Kubuntu 6.10) running on the latest build (3120) with more than 512 MB of RAM. I have 3 GB of RAM (Mac Pro), and I'd like Kubuntu to have 1 GB or 1.5 GB (which apparently is supposed to be the max), or possibly even 2 GB.

Does anyone know if there's a workaround or if there's a fix coming?

 

Edit the options for your Linux kernel in its bootloader configuration to add "agp=off".