Vid card problem?
My guess is that the problem you're running into has to do with the new video card first and formost, and possibly with a related problem with the new AGP drivers that would be needed for the motherboard.
It sounds to me like what is happening is that your system is booting up ok, but then failing to load X properly, which tends to cause either a complete lock-up, or just causes X to freeze. The trick is to boot the system without loading X.
There are a couple ways to do this. First, you could try booting up normally and then killing the X server when it freezes up at the blank screen (hit CTRL-Alt-Backspace to kill an X-server). Otherwise you can try booting into "safemode" (Mandrake normally puts this in as a LILO option). Finally you can manually go into single user mode in Lilo by typing "linux single" at the Lilo prompt.
The next step would be to fix up the driver that is used. Mandrake probably has some sort of utilitiy to do this automagically, but I haven't used Mandrake enough to know what it is. I do know that it can be done manually by editing the /etc/X11/XF85Config.4 file. In that file there should be a section which talks about your graphics card and would have some sort of line dealing with the driver used. If you change that line to use the "nvidia" driver, you should be good to go into X at least. After that you could probalby use some of Mandrake's graphical tools to fix up a few other things.