RAM will depend on the motherboard and CPU support for it. As far as I can tell, your system uses DDR2; so any modern motherboard and CPU will be incompatible with it.
Disc drives will depend on the interface with the motherboard. These should be fine as they seem to be SATA.
Power supply may depend whether HP uses a proprietary form factor and connections. For some prebuilts the included PSU doesn't use standard connections (particularly the main power cable to the motherboard).