I just wanted to share my RAM upgrade experience, since everyone is usually looking for advice (myself included).
My laptop is an HP 2000 model, they sell at Walmart for $350-$400. The chip is an AMD E-450 (only uses 18 watts), comes with one 4gb RAM stick, and a 500gb hard drive. Not the greatest processor speed wise, but this computer has served me well for almost a year of heavy use. I take online classes, and have done some 3d modeling/rendering with it.
The easy way to find your computer's exact model number is take your battery out, and it's listed on the inside.
So that is how you can go about ordering memory for your laptop. It was VERY easy to install. Just had to remove one screw, take the backplate off, remove the one memory card, and it has slots for 2 cards. Put the new cards in, put the plate back on and you're done. My computer recognized I had 8gb right away in the control panel.
So does it run better with 8gb instead of 4gb? Yes it is noticeably faster. Webpages load faster, I'm using Google Chrome. I tried 3d rendering and it also now renders a bit faster. The only thing I did not see an improvement on was running the game Civilization 5. The screen transfer is still choppy with that game, I think it must need a faster CPU. I read before that going from 4 to 8gb should be about a 50% difference, and that seems about right. It's not twice as fast (100%) but it makes the computer perform noticeably better. I definitely think it was worth the $40 for the 8gb kit.
The desktop computer I'm saving up to build will use an Intel i7-3770 chip and have a 128gb SSD for Windows. So obviously things like a fast computer chip, and a SSD to boot programs fast are unbeatable for performance. But if you have a laptop that has 2 or 4gb, and it can be upgraded to 6 or 8gb, I think it is worth adding the extra memory to make your everyday laptop a little "snappier".
You left out one crucial piece of information. You need a 64 bit OS (Windows or Mac OS) to take advantage of anything more than 4GB of RAM and you have to make sure your motherboard will support it, which as you pointed out, can be determined with Crucial's System Scanner Tool. or use of the Memory Advisor. On some laptops the model number is listed on the bottom of the computer.
Thanks Ken, that is correct and I should have added that you need a 64 bit version of windows to use more than 4gb RAM. It seems to me like the 64 bit version is more common these days, so that's why I didn't think of it, but a very important point. And of course Windows 7 Home Premium (64 bit version) has a maximum memory use of 16gb. Any more than 16gb and you need to get Windows Pro or Ultimate which support up to 192 gb.