Are you wanting to buy a new box and use the same hard drive in it? Better to keep the old drive in the old box and use "ghosting" software available from most hard drive suppliers to make a complete copy of the old drive to the new drive. That way you can have two computers with exactly the same info on them, and let the old be your emergency backup computer.
I personally use two computers at the same time with a KVM switch between them.
A good used computer can save you some money. I like geeks.com and tigerdirect.com for "whitebox" type computers if that's what you are looking for, but there are other good sites like newegg.com too.