So you currently have 512MB of PC2100? You could go to just a gig by buying 1 stick and save some money. I don't think you'll generally see a huge performance increase from 1 to 1.5 GB.
That seems like a lot of money for an AGP video card that you can't use in a new system if you ever get around to building one. I know you've gotta do what you've gotta do but if you could come up with another $250 you could have an overclocked C2D and possibly a better video card for the same $140 price point.
You have an early P4 (with the 400FSB) and most likely you have SDRAM, not DDR. Initially RDRAM was used in old P4 systems, but it became too expensive. So to cut costs manufacturers started using SDRAM which is considerably slower than DDR and A LOT slower than RDRAM.
Is your computer a homebuilt one or is like a Dell, Gateway, etc?
If I were you, I'd save my money and start from scratch. What you're upgrading is still going to be outdated and your performance gain (for whatever you use your computer for) will not be very noticable (with the exception of the graphics card).
For games, you computer is going to be getting to the point of bottlenecking. That AGP card is nice, but with newer games the rest of your system is going to lag behind. Unless you're not planning on games, then I'd recommend load on as much ram as possible.