Which Ram Configuration should I use?

This new build of mine marks my first venture into the world of DDR memory. (having only used PC-133 till now, I'm really moving up in the hardware world huh?
)
Here is what I have to work with:
2GB PC3200= one 1gb, (2X512MB)

Here is what I have to work with:
2GB PC3200= one 1gb, (2X512MB)