well it doesnt matter what size the the memory is (example: 2gb, or 4gb dimms) as long as they are the same speed and voltage then they should run fine.
They also need to be the same size
Two RAM modules, one 2 GB and one 4 GB, would be a little slower than two identical modules and would not run in dual channel mode. On the other hand, most people would never notice the speed difference.
On the third hand
, your system would be faster with 6 GB of mismatched RAM than with just 2 GB.
Otherwise, Jack Naylor gave you the best advice.