Memory configurations

Hello people,

Its a long time since I came here last, but I know you are all just as willing to help an old fuddy-duddy as you have always been.

I have a memory configuration question. I want to upgrade my laptops memory. Its a Fujitsu LifeBook AH530, but I dont think that matters for this question. It has two DDR3 memory slots. one is occupied with 4GB stick. I know generally the more memory the better, but will I suffer a performance hit if I add a 2GB stick to the spare slot (assuming the memory speeds are the same)? If I would suffer, then would I be better off having two sticks of 2GB rather than a 4GB + 2GB configuration.

I realize the best thing would be to buy some, but with limited funds and a spare stick of memory then I thought Id ask.

I also know that the speed might be affected by the speed of the slowest stick installed. More memory = a faster machine, but stick with slower speed memory installed = a slower machine (possibly). Is there a general formula that will tell me the performance gained by the mixing of speeds and size?

Thank you in advance for your help.