Your going to need to take out the ram you have, and buy 2x4gb sticks.. there is no way that i know of to add ram slots on to a motherboard.. the only way your going to do that is if you have a new motherboard with more slots..
if you do buy another motherboard with more slots, make you you buy EXACTLY the same ram as your old ones if your going to use them together.
if your sticking with the motherboard you have now, MAKE SURE,
firstly it can run 8gb ram
2nd make sure you check the maximum RAM frequency your motherboard can handle.
and lastly, check to see if your motherboard is DDR2 or DDR3 compatible.. if its both then go for DDR3 ram!
oh yeah, also, if you want alot of ram and your motherboard can handle 16gb ram, why don't you go for 1x8gb stick.. at least then you can add more if needed? ;)