I'm going to be fixing up an old box I had in the closet and running debian on it, going to be an all-purpose server for my own uses... but I wouldn't mind it being a game server or something... if it's got a P4 @ 1.8 GHz and 512MB RAM, would throwing a extra 1GB in it help any?
Yes, depending on the operating system you are running it might help more or less (Debian from my experience is not that resource hungry). Any time you increase the amount of physical memory in a computer, you should notice a significant performance gain. The performance gain will be less noticeable when you are running fewer applications, but will be dramatic under heavy usage. Just make sure you're motherboard supports the memory upgrade before you commit to buying anything. Some motherboards limit the amount of RAM per slot/Total per board, and place other such restrictions on adding RAM.