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It really depends if the OP is even going to reach the 2GBs with his current use. I think the bump from 2 to 2.5 is not a whole lot. If you are going to start swapping at 2, then you probably will be at 2.5 as well.

Samwise592: check to see how much blue and green is left in Activity Monitor.
I'd just go with the single 2 GB stick to max out out one slot.
 
I'm at school right now, (Mac not with me) so i'll check when i get home.

But i DO know that my page/outs are usually in the hundred(s of) thousands
 
I'm at school right now, (Mac not with me) so i'll check when i get home.

But i DO know that my page/outs are usually in the hundred(s of) thousands

Its actually the ratio that matters, not the total number. If the ratio is less than 10, you need (or will benefit from) more RAM.
 
noob question

How exactly do I check how much RAM I'm using?

And please don't say open Activity Monitor and look at Free RAM, as I know OSX grabs all the RAM available for its system caches.
 
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