PC4Lyfe :
I need to know your CPU and socket.
You can't Google HP Pavilion 550-150? Third link on the search results.
http://support.hp.com/us-en/document/c04790568
@red2481,
There's a problem with upgrading the motherboard. The OS that comes with the PC is a OEM version, meaning it's tied to the motherboard. You would have also buy another Windows Key for the new motherboard. Why are you wanting to upgrade the motherboard?
Also, for what reason are you looking for an aftermarket cooler? The i5-6400 can't be overclocked which is typically the reason people buy aftermarket coolers.