At the moment the best 'bang for the buck' is definitly not in the GPU PPD, it's in the Multi CPU rigs. High cost to start off with but PPD/Watt is alot better. Also if you've got 12 cores (reported by OS) or more you can get the 'new' big-bigadv WU's, which get you loads of PPD.
in terms of a 500 pound rig, the best bang for the buck is an 2600K (capable of regular -bigadv) or any higher performing LGA1366 CPU (the 6c/12t variants) if you could find one.
a GPU gets some nice PPD aswell however running it is more expensive than just a CPU. Do be aware that you will need some good cooling.
Ow, just a small disclaimer... the Pande group is constantly working on this project and the points that are awarded. In the future there might be a QRB (Quick Return Bonus) for the GPU's aswell, which may cause them to get a better PPD than large multi CPU systems. When and if this will happen is still unclear though.