depends of what you are doing. If you are running apps which can use more than 2GB of RAM, the 4GB of DDR2-400 might give you more performance. But this is not certain and even with 2GB of DDR2-800, the performance might be better.
If you are using less than 4GB, which is most likely, the DDR2-800 will perform faster for everything.
IMO, 2GB are more than enough for 99.99% of the population on the earth, regardless of what you do.