This is a general recommendation for any laptop, not just my beloved MacBook Pro: max out your memory. Buy as much as you can jam in there, and install it. Don’t delay. There’s no reason to suffer with slow task-switching, etc. I do this with every laptop I use and I’m always absurdly happy after I do the upgrade.
The RAM slam is particularly dicey if you’re also running Windows on OS X using Parallels Desktop. I’ve run Parallels in 1GB of RAM and I can say that 2GB makes a huge difference — the extra RAM means you can switch back and forth between Parallels and OS X in less than a second instead of like 10 to 20 seconds with 1 gig of RAM. (I should say that other than memory consumption I am totally loving Parallels; just for chuckles I installed Ubuntu on my Mac in addition to Windows Server 2003 and it’s all working quite well.)
Now, that said, it nearly never makes sense to buy laptop RAM from the actual laptop manufacturer, since they kill you on the markup. Buy it from a reputable aftermarket retailer like Crucial, or from a brick-and-mortar retailer who knows what they’re talking about. (If you walk in and tell them what kind of laptop you have and they can’t tell you what kind of RAM it takes, they don’t know what they’re talking about.) I popped by Fry’s Electronics in Palo Alto today and bought the memory I needed for a smidge less than the prices I found online, hooray.
One other thing. I couldn’t figure out how to determine what the factory memory configuration on my 15.4" MacBook Pro was without opening the case. This caused me to buy two RAM chips instead of the one I needed, since the default configuration Apple used on my machine is a single 1GB chip in the first slot and nothing in the second slot. This made me ecstatic since it made the upgrade much easier, but now I gotta go back to Fry’s and return the extra 1GB chip I mistakenly bought.
Update: Well of course there is a way to view your memory configuration without opening the case, duh.