I don't have this Laptop at hand right now, as it's my mother's laptop...
I have looked for D800 linux reviews here :
http://www.linux-on-laptops.com/dell.html
it looks like the D800 comes with a NVidia card. Can you confirm that ?
to make sure, do :
If you have a NVidia card, that's very good because you can install the nvidia driver and it will easily handle whatever resolution you want.
If you have a i855 chipset, look for i855wrap and i855resolution in google, I'm sure you'll find what you need. You'll have to use the i810 driver in your X11 config file, and you'll need to add a few lines to call i855resolution and i855wrap at startup with proper arguments.
I you really can't work it out, tell me and I'll look at what I had precisely done on my mother's laptop...