mixing RAM?

I am looking to add some RAM, and yes I actually need it, i have been reading and know it can take up to 16. atm I have 6gb wit one empty slot. Wanting to put a 4GB in it so I would have 10GB. I know people say its best to keep it all the same



I was wanting to add this







how well would it work? I have heard that its best to have all sticks the same size? I just dont really have the money to fill all 3 slots with 4gb ram