Why does it have to be inside the Presario case? If you're intending on buying a new motherboard, why wouldn't you spring the ~$80 USD or so for a new case?
It would actually help more if you stated what country you live in. If it's in the US are you in the continental US? (Determines where you can buy parts from and what/if warranties apply to you) It'd also help if you explained what you'd like to use it for. (Internet, Video, Games, etc If you want it for games you should say how well you want them to run.)
I live in Canada and I'm not in a city so it's a little tough going to a computer store and asking what I will need. Also, I will be using the computer for gaming and internet (obviously). I already have a PC case, but I cannot use it yet until I find a new motherboard and my hard drive can't fit into the PC case. I will try to post pictures of the inside of my computer, but right now photobucket is down. Don't worry though, I will post pictures soon to see what people will suggest what I will need.
I'm not very experienced when it comes to upgrading a computer and adding new things to it and finding out what will fit and what's compatible. Is there a possible way of sending in my computer to a computer store or something and let them upgrade it for me and they'll send the bill? Just curious.