The 5400 is the later addition to the AM2 processor range, initially it only went up to a 5000+ processor and then the range was expanded up to the 6000 with the LE range, the BE series and the Phenom quad-core processors. BIOS updates all round were needed quickly since none of the boards could see anything beyond the 5000 and AMD were shovelling the new processors out like there was no tomorrow.
The A8MZN-LA looks to have been used in a HP Pavilion A1600N and probably other variations too. One post on a HP Business forum suggests you can stick a 6000 processor in it. Therefore if its the same board a BIOS update from HP should let it see a 5400.
Edited by Neil Jones, 05 November 2008 - 05:37 PM.