I think that the difference would definitely be noticeable, but not huge.
Getting 1GB instead of 512MB mostly helps with level load times, and a little with the actual gameplay.
Getting PC3200 will speed level loads and gameplay up a bit as well.
I think the difference will be worth it, but dont expect your mind to be blown by a whole new computer feel
As for dual channel, I doubt you would even notice a difference. DC barely increases system performance (1%-5%). Still worth it, though, considering how easy it is to enable it (2 sticks of similar RAM).