If you're attempting to just keep yourself from falling behind noticably, $100 isn't going to be able to do anything. Upgrading the CPU would require a new motherboard, and you don't have money for both. Upgrading the vid card would be a poor idea - from your plan here, sooner or later you'll need to upgrade the mobo and CPU to keep up with the curve, and that will almost certainly mean PCI express - which would mean you'd need to switch over your graphics card when you do it. Memory is stalled for the reasons you've already said.
So, you're pretty much stuck with little stuff. Maybe a better case. Maybe a new DVD burner. Maybe a new PSU.