Upgrading my HP pavilion a6110n

If you're going to spend the money to replace everything in the computer, why not just start from scratch and build one from the ground up. That way you won't have that goofy looking HP case giving you a headache.

But as long as you're planning on replacing the motherboard, anything is possible. What mobo you buy will determine almost everything else you asked (RAM, processor, expansion slots). As far as the PSU, it depends how many drives, fans, etc. you will be running. And GPU depends on the measurements of the case itself. You may have to do some internal measuring and then compare those to the dimensions of the card you are intending to buy.

This has potential to be a good PC, but you're only saving yourself $50-100 by using a case you already have. Like I said, I recommend starting from scratch. But whatever you do, have fun.