as far as I know, changing display driver for X and resolution doesn't mean you have to reboot...only restarting X should do. anyway, to see what the problem really is (or to try to find out), do this:
1) when you've asked of username/password, or in other words, when the system still has not hanged up, press CTRL+ALT+F2 to get into shell. log in.
2) check some logfiles that should have appeared after last hangup:
cat /var/log/XFree86.0.log | less
cat /var/log/errors | less
cat /var/log/messages | less
(and optionally this
dmesg | less
and see at least the bottom lines for errors. if none are found, then press CTRL+ALT+F7 and you should get back to graphical screen..log in, and when it hangs up, try to press CTRL+ALT+F2 to get into console. then repeat step 2 to see the logfiles again...
if you see any errors in them that tell of graphics card, driver or screen errors (or others that belong into this problem), see them and post here if you don't know what to do with them or suspect they would help
also, you can try to change the driver back from the console. but still it would be better to get the problem solved, not just undone..
