Get a 500W PSU

Get an AMD FX processor entry level is like 100 bucks. (Aim for the 6100 if you can)

8 GB ram (Corsair Vengeance is my fav, the 1333mhz is only like $40)

A 7750 or 7770 vid card is only around $150 but will run Diablo smoothly and can be upgraded if you want to spend a little more. (7850 for $250/7870 for $350)

A simple 1 TB HDD or get a 60 - 120 GB SSD if you'd rather shit run fast and load fast, im too impatient to have a PC without a SATA III SSD, OCZ Vertex runs well can pick up a 60 GB for around $100)

All you need is a case, and a mobo, you can get a mid tower for around 100 bucks more or less depending on style/looks and then the mobo should be around the 70 - 100 mark.

Obviously a CD drive for 10 - 20 bucks too

This is what i'd get anyways.

If you need to cut costs get 4 GB ram and only a 250 - 500 GB HDD.