Maybe Mandrake didn't install the drivers for your video card. Can you boot into a text console?
If so, you could try downloading ATI's drivers for the Radeon 9000 (which your card is equivalent to, I think) at http://www.ati.com/support/drivers/l...e&cmdNext=Next
and installing them, following the instructions on the link. It has to be done through the command line, but it shouldn't be too hard.
But even if the ATI drivers aren't installed, I thought there was a generic driver that at least allowed it to run X. Perhaps Mandrake did not auto-detect your graphics card.
If you get any different errors after installing the drivers, post them here.
Good luck with Mandrake and with Linux in general!