micro-atx is a sort of mini version of the ATX standard. A micro-atx motherboard will have a max of 1 AGP and 3 PCI slots and generally are geared more towards the OEM market and resellers since they usually sell themselves off as a motherboard with a lot of integrated features.
Micro-ATX power supplies go in micro-atx cases. However, some micro-atx cases can take standard ATX power supplies (a must if you get a micro-atx athlon system). You'll have to check with the cases on that.
ATX is the normal, everyday, full size motherboard/case form factor that most motherboards adhere to. An ATX case can mount a micro-atx motherboard but a micro-atx case cannot mount a full ATX motherboard.
I don't know what the MSI board is...If it has more than 3 PCI slots it's most likely full ATX. If it only has 3 PCI slots or less it's micro-atx.
Gaidin