How can I get a large virtual desktop?

Fri Jun 16 08:29:54 PDT 2006

Running gentoo linux
xorg-x11-7.1 
I'd like to have a large virutal desktop that in previous hardware and
gentoo installs I was able to get by setting something like 
 Virtual 2048 x 1600
I recall doing something with xvidtune to get those numbers but with
my current vid card (MSI FX5700LE) when I run xvidtune it just says:
 Vendor: , Model: 
 Num hsync: 1, Num vsync: 1
 hsync range 0: 31.50 - 79.00
 vsync range 0: 40.00 - 150.00
 Video are not settable on this chip
Any one have an idea how to do this?