Most people don't like their desktop resolution to be messed with. It screws up icon alignment, and I just don't think it's worth the trouble. Good luck though.
Additionaly some people have LCD moniters and other techy stuff with fixed max screen resolutions....more then icon alignment can get messed up if you try to force a larger resolution...
And if you have an 800 by 600 window...why not just check to see if such a resolution is available in fullscreen, then ask if the user wants to go that route?
You can then use one of the many available Blitz3D basied GUI systems as a baseis for your own windows like user control system if you still need such features in fullscreen.