1: not hard. bunch of screws and connectors. Your problem is that
a: the HP case may be proprietary and another non-hp board may not fit
b: the operating system is tied to the HP motherbd and will not transfer to a non-hp motherbd.
3: if you have a pcie GPU then no. If you have an AGP gpu then yes you will need a new one
Thanks for your comments. I will likely leave it the way it is.