This is a (very) short list of games I have played recently which are online, multiplayer, and have massive worlds. Also, they all work on my computer, which has a very old video card, not suited for hardcore gaming. These games should all work for you as well.
Runescape -
www.runescape.comOkay, I have to mention this first, because it's pretty much the only Java-powered MMO game I ever played, and it plays well on any system that supports the latest version of Java. Also, it requires absolutely no downloads, meaning you play it entirely on a single webpage.
MU OnlineWhile I never played MU Online before, I started playing it via
the Forsaken server today, and I must say, "Wow, this is like Darkstone, but it's online, and it has better graphics!" If you can wait for the updates to complete, this game is worth checking out.