I'm sorry to say it will never run really well unless you change just about everything and if that's the case it will be much better for you to buy a new one which will cost the same as upgrading all the components. I was in the same boat but had a slightly better one than yours, but I suffered for 3 years playing GWX really badly.
I calculated how much it would be to upgrade and it would've cost the same as building myself a new one because fast (relatively speaking) AGP graphics cards cost a fortune. Also old memory sticks do as well. And you are always limited by your processor and mobo which you cannot upgrade.
To get the game running you would need at least 1gig of ram and a half decent graphics card. And then it will still play badly. I'm afraid to say that will set you back about a 3rd of how much it would cost you to buy a new good PC. Hence why I didn't bother.
I would recommend 2x1gig stick of RAM and something like an ATI Radeon X1950 pro AGP or equivalent graphics card. Your processor will limit you graphics wise. If possible at least an 80gig hard drive...40 gig is tiny. That's the bare minumum in my opinion. Good luck.