Keep in mind that if you upgrade your motherboard you will need to purchase a new license for Windows.
Keeping with your current motherboard I'm not sure how much of an upgrade you can do CPU wise. Download CPU-Z and look at the "mainboard" tab, you should either be running an AM2 or 939 socket.
For RAM you can get DDR2 667 or 800 MHz.
In my opinion cases are personal preference, you're the one who has to look at it.
Can't go wrong with a WD Black hard drive.
(assumed system specs
When you say "minor gaming" what games are you looking at?
My current system is much slower than what you're currently running and can manage fine with WoW, EVE, and a number of Steam games on the Source engine.