Need more info about your current system before we can advise you on what way to proceed. I would say do not get a 2.4C first though. You should get a mobo first if your 1.5 is a socket 478. If you have a socket 423 system, then you need to get both the mobo and CPU at the same time. You should get a Canterwood/Springdale mobo if you plan on getting the 2.4C. Your RAM will get you by for a while, no problem. You'll just have to run the memory bus at a lower ratio is all. You'll take a little bit of a hit on memory bandwidth, but it'll be a huge gain over what you currently have.
EDIT: Forget what I said about getting the mobo first. The Canterwood/Springdale mobos don't support the Willamettes. You'll need to get the mobo and CPU together. Your RAM will still work ok, though.