You may have to config it for your screen.
Try typing: dpkg-reconfigure xserver-xfree86
This will take you to a screen that will allow you to input your hardware requirements. Before you do look up the horizonal and vertical refresh rates so you can do the advanced install. Otherwise go with the medium setting and best guess.
EDIT: Once you get it configured correctly the startx command will just take you to a dull grey screen. exit this then run
apt-get install kde
apt-get install kde-core
for a slimmed down package to update later or,
apt-get install gnome
which is the one I like.