The main difference is that full ATX motherboards have a maximum size of 12"x9.6", and Mini ATX boards have maximum dimensions of 11.2"x8.2".biggest differences between motherboards of different ATX-style form factors are dimensions, and the placement of mounting holes. This means that most cases are "downward compatible" with the smaller ATX variants, as long as they have provided appropriate sets of places to put mounting hardware. If a case will support installation of a full-sized ATX motherboard, it may also support a Mini ATX, microATX or FlexATX board as long as the designers of the case have taken the smaller form factor variants into consideration.
the mobo that is in there at the moment is about 9 1/2 inches square. I dont think that i could get anything wider in as it would hit the back of the HDD and CD's. There does seem to be some space below the mobo, with some additional spaces at the back that are still covered, but look as if they could be pushed out. So presumably I could put in a longer board.
simple checks such as the connectors for the mouse or keyboard are they blue and green (ATX) in the socket are they at the extreme top (ATX) under the power supply or are the in differant places (AT) download " Belarc Adviser " or " Aida2 " or " GetSysInfo " either will give a full list of components and devices software etc.