Basically the conclusion is that you want to run at 1:1 and some things will run better with more ram, and some will run better with faster ram.
For instance, my computer running at 2ghz (10x200) is plenty fast enough for Doom3 at Ultra Quality, but UQ requires over 500 mb of ram for texture memory. The result is that my computer will swap (pause for a second and read the hard drive furiously) when it needs more memory, but in between swaps it runs around 50fps. In that particular situation, im sure if i had 1 gb of 2700, it would not swap and my system would still be fast enough, resulting in Ultra Quality at a good frame rate with no swapping.
DoomIII is the only thing i have seen need more than 512mb ram for though (in what I do), so i'd go for the 512@200, personally.