You probably only need to upgrade the motherboard and processor if your existing components are compatable and you don't plan on using the old PC for anything. These often come bundled together potentially saving more money.
Scan's Today Only offers usually have a a fairly high spec motherboard, processor and memory bundled together for less than the components listed in the original post. They currently have a midi tower case and 550W PSU for £38.72.
Computer hardware and software at amazing prices, available online from Scan Computers UK
I wouldn't spend much on a new case unless you plan on using it in your future PC.