it's because you're mixing and matching some odd combinations of RAM there. There's no point in putting in that PC2100 since it'll just severly slow down the PC3200 sticks.
I know with the MSI boards you have to put the sticks in certain slots and depending on if the ram is a double module makes a difference in where it goes too.
If you fill up all 3 slots it can only run it at PC2700 speeds so there might be some conflict being that you're trying to put in 3 different speeds of different sizes of probably different manufacturers....
The board might be trying to run dual channel with the two PC3200's and then you throwin in another stick is throwing it off or something along those lines.
You should put the 2 256mb sticks in slot one and two and the 512mb of PC2700 in slot 3 and see if that works.
BTW: What brands are you using? Have you visited MSI's website to see their compatibility list and their RAM tables that tell you what combinations give you what speeds and whether it's dual channel or not.