4 gigs is fine for average use. If you're using Adobe Photoshop, doing HD video editing or playing demanding 3D games then 6 gigs could be useful. If you get the RAM in 1 DIMM slot, then you'll be able to expand memory by adding a second memory module in the second empty DIMM slot. If you get 4 gigs in 2 DIMM slots (two 2 gig DIMMs, one in each slot), then you'd have to discard one to upgrade memory.