Personally, I think you're crazy for trying to use Vista on that thing.
Really, switching to XP would be a wise decision.
When it comes to upgrading, it is far easier to go about updating graphics, but because your processor is so bad I would almost be willing to say that you need to buy an entire new mainboard.
Just make sure you stay the heck away from local stores and buy everything from trustworthy online shops. (Newegg, Tigerdirect, etc.)
If I were you, I would spend the extra money and just get a new PC; though it might be easier to rip everything out of your current case and refill it with better parts.
Just make sure you have plenty of room and what not.