Yes these boards will fit in your case. All you will have to do is remove the old I/O panel and install the one that comes with the new motherboard. The I/O panel will snap out. An I/O panel is the metal piece of the case that the keyboard and mouse ports come through in the back of the case...
"There are many micro ATX motherboards you could buy but moving from one socket 478 mATX to another socket 478 mATX motherboard will not be much of an upgrade"...
Cinders is dead wrong here! The crippled bios of the original HP motherboard, will be very out done by the simple Biostar bios. The performance of the Biostar boards with the DDR2 and the PCI-e will work great compared to the old motherboard. I have already said that you will need to purchase DDR2 memory and a PCI-e video card, NOT AGP! Look carefully at the Biostar motherboard specs and buy DDR2 memory for that spec. You might upgrade your power supply too