The manual is here:
http://us.dfi.com.tw/Support/Downloa...YPE=MB&SITE=US
1. The 900 MHz Athlon is 100 FSB, and the 1.2 GHz is probably a 133 FSB. You need to remove JP10 for 133 FSB operation. That's the jumper between the floppy connector and the ram slots.
2. You do need a bios update, download the OFFICIAL bios update here:
http://us.dfi.com.tw/Support/Downloa...FLAG=A&SITE=US
Get 2001/11/05.
EDIT: Hmm - you have 2001/03/22 already - that was SUPPOSED to fix the ram issue. I'd update anyway and go from there. You may have ram with the wrong component density.