Motherboard, fans, ram slot pieces, pretty much it.
I was considering at one point buying a bare Mac Pro like what you are looking at. Turns out for the RAM risers alone was $100 for a set. Then You have to spend $40-60 on HDD sleds, then a stock 7300GT graphics card then find a PC one that is flashed (or drop money on a official Radeon 5770). It's one big money pit. I didn't buy that one for $150 just for the reason that it was going to cost more to be put into stock form than to buy a machine that was already stock.
----------
I recommend an ASUS gt210. Cheap, works well as a stop-gap for a newer card. Its $30 on Amazon, sadly no EFI boot. PowerBook medic has good RAM for the Mac Pros.
Those nvidia cards are so low end like the Radeon 5450. I have a 5450 in my Core 2 Quad desktop and the thing struggles with Minecraft and the Sims 3. I've owned both and they are just junk. A much better option would be a flashed Radeon 5750 or the official apple cards.