From what I've seen, with that motherboard, you'll need DDR2 RAM. For this, I'd be thankful, as DDR3 is still a bit more expensive.
RAM is sold in sticks of 4gb, but it isn't the cheapest for sure ($400 and greater for a 16gb set).
http://www.newegg.com/Product/ProductLi ... x%204GB%29And as mentioned by Kilroy, you'll need a 64-bit OS, which I'm assuming you're already using.