How can I get a large virtual desktop?

[reader at newsguy.com]

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?