1) usually it doesn't void the warranty. Most of the time only hard drive upgrades/cpu upgrades/any mods like painting or LED lights will void the warranty. A safe thing to do is just save any RAM you're taking out, and in the event that you want to return your laptop, just take out the NEW RAM you upgraded with, and return the OLD RAM. I've heard people do this with their hard drives because the warranty only works with the original hardware.
2) Basically there's 3 kinds of RAM. SDRAM for really really old old laptops. Probably, you need DDR RAM or DDR2. What kind of model do you have? You can just check the internet, but if it's a newer model withOUT sonoma, then it's DDR PC2700. If it is the new sonoma chipset, it's DDR2 PC...something I forgot.
So just make sure you get the right PCxxxx. Also, I've heard some laptops have smaller memory than others.... So far I've only handled 200 pin.. But I've heard that some had 184.. Just check on the internet if you're not sure or make another post with the laptop model. Hope that helps.