run CPU-Z, and you will know what ram you have
the link you posted have DDR2 ram.
if you think about it, most mobo have the most of 4gb, and the computer might have 4 x 1gb ram, or if it has 2 slots, then 2 x 2gb ram.
I barely seen any factory built computer leave any space for another stick or 2 of ram unless it has like 1gb. So i dont know if you can add your 2gb of ram.
plus 4gb is enough, its not effective unless you have an x64.
The only way to be able to use up over 4gb is mainly if you made it a server, and the processor might bottleneck the 6gb and limit the amount.