I have used a few diff. pads with my g500, but always seem to go back to my Steelseries 4HD. Great for FPS and just about any game I play. It is about 9.5" by 11.5" and costs in the $25 range on any given day at your favorite etailer.
As far as what dpi is best, that can only be determined by you. I vary my dpi settings from 400 to 1600 depending on the task I am doing or the game I am playing. Play with the settings until you find what works best for you.