well, if you don't plan on upgrading the CPU, I'd say RAM first. But it really does depend on what you want to do with it, what you'd like to improve, etc. Oh, and definitely upgrade the power supply.
Edit: I thought the RAM looked a bit obscure. I haven't seen how expensive it is. You could maybe go the other way around, and scrap the old PC for parts for a new one. You could probably use the hard drive and maybe the optical drives.