heh, so the computer doesn't even boot?
Since you have a G400 I'm willing to bet that your motherboard might use jumpers to set the FSB and clock multipliers, if your FSB(front side bus) is set too high it can cause the AGP to be at too high of a clock speed thus causing it to crash...
If you just mean the the image on the screen freezes, that could also be the system RAM not liking the speed it's running at, make sure it's rated for the FSB you are using
Basically, I'd check to make sure it's running at the proper FSB for your motherboard and ram, there are many other possibilities as well, it would probably be wise to post your system specs and a more detailed account of what happens