Yes you can upgrade your Hard-disk drive and your RAM easily.
I think this is the only components you can upgrade. You can't upgrade your CPU ( well yes, but you'll void the warranty of HP ) and you definitively can't upgrade the graphics chip.
In other words, can I physically upgrade the ram and hard drive myself purchasing from newegg or someplace other than hp.com? I am still configuring my system and want to know what components I can find cheaper elsewhere, other than OEM. What other components besides ram and hd can I upgrade?