Unfortunately it doesn't look like you'd just be able to upgrade the CPU alone - your current motherboard is a little strange and doesn't seem to allow that. I would strongly recommend at least considering an entirely new build, something like this: http://pcpartpicker.com/p/Bz3ZD3
If you do decide to go with that build, just keep in mind you'll need to buy a copy of Windows 7 or 8.
An improved GPU might improve performance in Minecraft, but your CPU is pretty slow. Minecraft tends to be more heavily CPU based and so it might be a better idea to upgrade that if at all possible.
Unfortunately it doesn't look like you'd just be able to upgrade the CPU alone - your current motherboard is a little strange and doesn't seem to allow that. I would strongly recommend at least considering an entirely new build, something like this: http://pcpartpicker.com/p/Bz3ZD3
If you do decide to go with that build, just keep in mind you'll need to buy a copy of Windows 7 or 8.