The first question you need to ask before upgrading your RAM is: are you currently running out of RAM? Do you truly need more? Honestly, the average person doesn't need more than 2GB of RAM (maybe even 1GB of RAM) unless you are using memory-hog MS Vista. OS X and Windows XP need less RAM in general...UNLESS you are running memory-intensive apps like Photoshop or opening two dozen windows at once. Whenever you edit an image in Photoshop, it eats RAM (the larger the image, the more RAM). If you are merely surfing the web, watching videos, and editing Office documents, you're surely not going to need more than 2GB.

If you aren't already running out of RAM, you won't notice the slightest difference in performance by adding more RAM. It's like increasing the size of your car's gas tank to try to increase the range of your car, when the longest trip you make is to the grocery store. Having a larger gas tank won't make any difference for that sort of trip.

While RAM may be cheap now (really cheap), it's also a waste of money if you don't need it. I'd stop at 2GB unless you really do need more.