Difference between 2G and 4G RAM for MBP?

More RAM means that more programmer can be opened without slowing the computer down. It doesn't really boost processing speed. If you use programmer like CS3 or Final Cut or any other programmes that requires a lot of power than upgrading RAM will help a lot. Also third party RAM is a lot cheaper. You might save as much as $500 or more. Ramjet is a good third party Apple supported brand that doesn't void Apple's warrenty.

I ordered the 4gb ram upgrade from crucial.com. It took maybe 10 minutes if you go slow like I did. In the MBP manual is a section on how to do the ram install.

+*Thanks for the advice. I've noticed there are Apple RAM sellers on ebay as well. If I buy those RAM instead, how should I install it? I'm assuming if I open the book myself or by any other unauthorised 3rd party, it'll void the warranty?*+

As a general rule... I would stay away from purchasing memory on eBay. Memory is one of the most important components in your computer and quality memory is important to ensure you have a reliable and stable system. There is no such thing as "Apple" memory. You can purchase compatible memory from any reputable vendor (Crucial, Kingston, PNY, etc). All of which will be new, with excellent warranties, and significantly cheaper than purchasing directly from Apple.

The memory in your MacBook Pro is user replaceable. You will not void your warranty by replacing it. You should hold on to your old memory just so you can use it to trouble shoot if you have problems down the road (so you can eliminate memory as the cause). It's generally a good idea to put the original memory back in if you need to send it in to Apple for repairs. But so far as your warranty goes... changing the memory is no different than changing the battery.

Here is a link on Apple's instructions on replacing your memory:

http://docs.info.apple.com/article.html?artnum=303491

You can purchase compatible memory from any reputable vendor (Crucial, Kingston, PNY, etc).

I used to agree with you. Then Apple complained about my PNY and KByte RAM.
http://discussions.apple.com/thread.jspa?threadID=977038&tstart=0
(The letter also said that if I had future problems and was stilling using the 3rd party RAM that AppleCare would not cover it.)
So I would stick with memory of the type that Apple uses. Samsung is the brand I know, crucial.com is the website I know, and I think those are mutually exclusive. 🙂 crucial.com is quite inexpensive. (Last time I checked, it was about US$130 for 2x2GB, not the US$500ish that Apple charges.) I think others on the board have recommended e.g. newegg.com. I would avoid PNY. Don't even think about getting Corsair memory.

All that statement means is that the RAM you used caused the problem. It does not mean that you "can't" use that memory. Memory from any manufacturer can fail. This is the main reason I suggested that you should keep your original RAM for troubleshooting problems. If you had put back your original RAM, you would have realized that the RAM was the problem. In general, PNY produces excellent products. Even the best manufacturers can have a bad chip now and then (I actually had a bad 1GB module from Crucial).