On a 500$ budget thats easy
semperon cpu 35   (Best mining cpu)
pos case         25   (buy any atx case)
hd 7850         200 (GPU)
PSU               150  (750 watt)
hard drive (sata)       25    (any mem works)
ram                         50   (4 gigs)
mobo                      75   (am3+ slot)

MIND YOU YOU WOULD NEED TO USE LINUX
AND IT would need to be wired in, not wirless
also if you would want to expand you would need a new gpu
OR buy a rig from me. Ill build one custom tailored for you with al mining software loaded up and ready.
We can work out price pm me.