The board will actually support 8 gigs(2 x 4gb). But ya... like spadam says it only has two slots. So you'll be tossing what you have in there now.
And you need old ram, DDR2. There will be lots of used dims on eBay and 2 x 2gb for a total of 4 gigs would be fine. I don't think were many 4gb sticks of DDR2 available; not like today with DDR3... 2 x 4gb is pretty much standard.