Yeah, thats most likely the cause of the problem. It was probably configured for the video card in the orginal machine, and is having trouble finding a non-existant card in the new machine. Usually, though, you would have to do some reconfiguring by hand, but maybe Mandrake has some fancy autoconfigure thing I'm not aware of.
Could also be that you didn't tell it to start in GUI mode by default during the install. Might be able to fix that by editing /etc/inittab and changing the default runlevel from 3 to 5 (I assume Mandrake uses those semi standard runlevels, but a bunch of distros I've tried recently do not, so your mileage may vary).