Go to Office Depot. I just assembled a desk (SKU #154198) with way more space than I ever had in the dorms (L-shaped, 60" sides + hutch + kbd tray), was under 100 bucks including the hutch.
Firstly, you should make sure that the motherboard fits into the PC case. Motherboards are available in different sizes such as ATX, Micro-ATX, and Mini-ITX. PC cases are dimensioned accordingly. A ...