GPU , you could try Groestlcoin and see if it rate of return is worth it.

CPU only ,  I would not even mess with, as it may shorten the life of the processor thru overheating

Your Best bet is one of the PoS coins like ZEIT or Noblecoin or Evergreencoin
The amount of coins per block matter more than the hardware.
Plus with these you can run it only at night so it does not tie up your CPU/Memory during the day.