My best suggestion would be to go to
http://www.nforcershq.com and hit their forums, they've got separate ones for each manufacturer (MSI, ABIT, ASUS, etc). Post their regarding your problems (or read, they might already have it covered).
Usually, they nforce2's like to have the single dimm in the slot closest to the cpu, but that isn't 100% across the board. If it works in Dimm 1, and not Dimm 3 (or vice versa), I'd email MSI's tech support, you may need to RMA it.