Four. You definitely notice a 100% difference in amount over 25% in memory speed - which isn't really noticeable anyway.

That mainboard uses normal DDR2 memory.

 

definitely the 4 gigs. the mhz difference shouldnt make much of a difference, and the added 2 gigs will definitely make up for it anyway. think of it like this: with the 2, you can move data faster but have less space to move it to, so it would have to move data in and out more often. the four gig though, moves data a little bit slower but has a LOT more space to hold data. in my eyes, the 4 gig would definitely be the better choice