I assume you already disabled your onboard graphics adapter in the BIOS, or set in the BIOS that the first graphics adapter to use is your PCI card?
If so, then you should boot your machine and when you are in the GRUB menu just type init 3. On the login screen then login as root, and start sax2 by typing sax2. You should then be able in Sax to select the mga driver and the screen resolution you desire. Then reboot and it should work!
Admo