If you have a site that only for hosting video/music/file sharing file purpose, and the main server has the script and everything else, does I need to upgrade the Ram besides 100mbps? Does 512MB or 2GB really matter when multiple connections are connected to the server? I plan to buy a new and wonder if it's worth the Ram upgrade.
depending on how everything is setup you can use allot of ram (caching) or very little, i would go for around 1gig to start out if you need more you can always upgrade to 2gb or more.
As ndreamer said it toally depends on how much you're caching. In an apache environment you could turn off keep alive and tweak the number of spareservers and things of that nature. It also depends on the web server if you're serving just files you should look at something like lighthttpd opposed to Apache as it can handle a lot more connections due to lesser ram requirements.