So I'd just like to make sure all these parts are compatible with
eachother and with my desktop? Are my parts good for hosting
a gaming server? Any parts you would recommend instead of
these? Any other info?
You aren't going to have a lot of luck trying to replace the mainboard in a HP branded slim case. I wouldn't even bother trying. Either make so with what;s in that computer now, or toss it and get a new case.
Do you have the system requirements for the game server? What game is it? You probably don't need as big of a system as you would think.
I say start from scratch too. You could possibly use some of the parts from the PC though. At least get a different case to put it in. That will be much more upgradable for the future. I also recommend shopping at www.newegg.com for parts. If you give us some specs and budget we can help you come up with something.