4 x 240-pin DDR2 DIMM slots Supports Dual Channel DDR2 800/667 MHz non-ECC, un-buffered memory (Max 8GB) There you go: max 8 GB, get that if you can affort, type is 240-pin DDR2 800 OR 667 MHz non-ECC, un-buffered memory. And if you get the max 8 GB, buy 2x 4GB sticks, so you can install it as dual-channel (works better), or 4 x 2 GB Sticks, which ever is cheaper. Just look how it is installed now and put the sticks on same slots. I bet your lag is because of low memory, if you have programs running on background...well you get the point