MacBook – Apple hardware: ram on MacBook Pro

Getting a performance boost depends on what you actually do with the thing. Run Activity-Monitor (Applications>Utilities) to see how much of your RAM is used. Most Mac stuff runs well on 4GB, but if you run memory intense stuff (Parallels-Windows, Video Editing, editing large pictures with photoshop) you should see an improvement. But even if you don't need the RAM, you should not see a performance penalty (i.e. it may or may not run better, but won't run worse).

Unless your computer is already slowing down to a crawl with the current configuration (when doing memory intense work), adding 2x 2GB should be enough (giving you a total of 8GB).

So, given your configuration, I'd add 2x 2GB to the free slots and keep the memory that is already there in the other slot.

It's like upgrade your house to have another room. Wether or not it justify's its cost depends on wether you use it or not. In general, I'd say 4GB is the minimum to go by these days, I'd opt for 8GB for most non-casual users.

Considering how expensive a MacBook is, I'd suggest going the extra mile to ensure your purchase lasts you well into the future. If your model is not a retina model (I.e. the RAM is user serviceable), I'd strongly recommend a 2x8GB kit like this one. At $130, it's only $30 more expensive than the 8GB Apple offers, but is DOUBLE the memory. (it's faster too, but your MacBook will bottleneck the speed difference, so it won't actually count for anything)