Now I Wanna Sniff Some Glue

I found myself in a situation where I was looking at upgrading the memory in a not quite bleeding edge name brand PC to give it some more years before the scrap heap. But, how much memory can it take and are there any other limitations I have to keep in mind? I don't build PCs nearly often enough to know by just popping the hood and peeking inside, like some seem to be able to do. "Oh, those DDR slots obviously take only up to 1GB each...". But I've built enough to know that motherboards usually come with good manuals, and those haven't ever failed me in a significant way. But this box is different.