but it depends on components and applications ---> it is hardly the memory at any speed which is the slowest part of the system. Say if you use photoshop with large files, it may be better to have 4 gigs of the 400. Just for simply stuff... office and web browsing the 2 gigs of 800 might be better.
we could also get into the whole 32bit OS not seeing all 4 gb too. There is hardly ever a clear cut answer to such a simple question
as far as i can say is that in applications that require higher ram, that is in quantity, will run better, with 4gb, than the 2gb. because just look at the previous ram speed tests. it says that ram speeds make less difference. if u have am2 socket processor then ddr2 800 will make much bigger impact. for core 2 duo, even now ddr2 533 ram is very good and more bang for the bucks. u wont see any difference in surfing or these small uses.
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.