adding RAM to macBook Pro

You only have two slots, so you would have to replace one of the 1GB modules with a 2GB one. It'd be better if you did that to both though, with the whole dual-channel thing...

 

Your MacBook Pro holds a maximum of 8GB of RAM.

Currently, it probably has two 1GB sticks of RAM installed. Based on this, you could purchase a 2GB stick to have 3GB total. Although because Macs have a feature called "dual channel" memory, to get the most out of the RAM, you want each slot to be filled equally, meaning 2GB in each slot, giving 4GB total.

It's not required to do that by any means, but just an added bonus.

You can find cheap RAM on Crucial.com, which is pretty much the place everyone goes to get it at good prices.