Make sure your motherboard is an ATX format - most common these days. For an
ATX motherboard, you need an ATX case - so that the *** holes in the
motherboard fit the holes inside the case.
Since you don't need a new power supply, buy a case without one.
PC case prices vary from $20 to $200 (plastic, steel, aluminium); just
google for "computer case sale" or something. You can also should google
"computer case reviews" - at least you see how they look inside.
I have a Cool Master - the Wave Master - that sells for $150 or so and does
a good job.