At least with all that RAM you never have to worry about it not being enough, and it'll keep you're PC running longer than if you used 512. If you can afford it, why the hell not? I can't... I have 512 lol.
I have 2.0gigs of DDR ram, and I don't use 50% of mine either. But the only thing that ever stays open are ICQ, Yahoo (Not Dead), AIM, MSN, Gmail, and my apache server.