I'm not 100% certain on this; I know its true with Dell computers and I'm pretty sure with HP too...you can't upgrade the motherboard without a new power supply.
If I am right, like Dell, HP makes their own motherboards and power supplies, which are incompatible with standard motherboards.
Also, you would need a new copy of Windows.
The first motherboard link is working, but did you notice it comes with a processor? The second link is broken.
Do you plan on doing any overclocking?
CPU: Core2Duo E6750 @ 3.6Ghz GPU: Sapphire 4870 512MB
Mobo: ASUS P5E RAM: 2x2GB Corsair XMS2 w/DHX
HDDs: 1x74GB Raptor; 2x500GB WD5000AAKS
Display: Samsung 226BW Sound: X-Fi ExtremeGamer
OS: XP Professional SP2, Vista Home Premium 64 SP1