it depends. i know mine has a port for upgrading it. Some onder ones you have to take the whole bottom off to get to the memory. so if yours has a post for it. it most likely is ok, but if you have to take the whole bottem off, it most likely will void it
Depends upon the make and model. If it's a newer laptop, then like Brian said as long as it has a RAM cover that is easy to remove it is probably ok. If it's a desktop it really varies from brand to brand. Check your manufacturer website to make certain. If you can't find the exact answer then sent them an email.
Again, it varies from brand to brand. I know that on desktops they used to put a tamper evident sticker on the edge of the case so they could see if you'd ever take the side off, but now-a-days they are more lenient. Simple computer upgrading is becoming more or less common knowledge, so now many warranties are not voided for replacing/upgrading what they call 'user serviceable parts'.