Anything that's somewhat recent will work. I usually shoot for celeron or better with Intel or a Phenom or better with AMD (Athlon or Sempron would probably work just fine). The only issue with really old cards is they may not have the lanes to handle 5 PCI slots. Your motherboard has them but the CPU doesn't know what do with them because it's so old. Still you can easily find a suitable CPU for around $20 on eBay. No need to spend more than that.

470 pulls around 100 total and gets close to 200. Yes that makes the 460 look more efficient but you will have to build more rigs to hold the same amount of hashing power. This becomes more expensive as you have to buy CPU, Memory MB PSU for each one. 470 or 480 is a better choice for that reason alone.