I really need a new computer, but I hardly do anything on my PC at home anymore. Usually when I am off, I go out to play disc golf, and I tend to get a little stir crazy if I am at my PC for too long. That, and I do not want to spend the mass amounts of money needed to keep my PC able to run games at a decent framerate. My desktop at work is a monster, so I do some processor intensive stuff there (since it would be mainly for work, anyway), and my office gives me a laptop, too, so I use it at home when I do use a PC at home. About the only thing my home PC does is download music and act as an SNES/N64 emulator when I do feel the need to play games.
Usually, I would go for the PC, but, in this case, since you are only going to throw about $300 at it, I would go for the keyboard, then save up some additional funds for a better spec'ed rig.