Computer memory upgrades for ASUS M4A785-M Motherboard from Crucial.com
Above link shows all compatible RAM at Crucial.com using their configurator for your motherboard model.
Take your pick from that list. You can buy from Crucial directly or make note of the part numbers and buy that RAM at the dealer of your choice.
There are several speed choices in that list. In your situation, I would just get the standard PC2-5300, Non-ECC:
Priced at 49.99. You may be able to find that same part number at a lower price at another retailer such as Newegg.