Wait, are you saying that it only shows up at 800x600? That means you don't have the proper video card drivers installed.
Start here.
Press Ctrl-Alt-F1, type /etc/init.d/gdm stop, install your proper video card driver (ati, nvidia, etc), then run /etc/init.d/gdm start
This will get you to the 'normal' installation. Then rerun the same steps above AFTER the installation, since it runs from a RAM disk.