on my laptop I use

xrandr --output LVDS1 -mode 1280x800 --scale 1.5x1.5

to rescale my 1280x800 screen to 1.5 times the native
resolution. Pixels then get inerpolated. Letters etc get to
be a bit more blurry than usual, but it works fine. I always
use it. Don't know if this works on the n900 (but let me know
if it does). Note that you have to replace LVDS1 with the
name of your display (run xrandr and see output) and you
have to use 800x480..

josx