According to Wikipedia, there is no such thing as DDR2 800Mhz, fastest is 533 1/3Mhz when looked by IO bus clock, so 800Mhz should be DDR3.
Now of course it is possible that memory in question is DDR2-800 which is 400Mhz and data rate of 800MB/s
Then there is of course memory clock, which can perfectly well be 200Mhz for DDR, DDR2 and for DDR3.
I'm not all that sure that everyone here is talking from same thing at all, but it is true that at some situations memory can be a bottleneck, however I think that textures are at videocards memory when they are loaded, unless display adapter is some kind of turbo cache model that has not own memory or at least not much, but those are usually horrible slow I believe.
So with small amount of graphic card memory it is better to limit texture settings, when all textures fit to cards memory performance is lot faster than when system memory is used as extension of cards memory, I don't think there is such fast system memory that would not kill the performance.
For example I have only 512MB of video card memory, so I cut down shadows and texture quality so that textures would fit to card's memory.
I have seen posts were people are complaining their cpu or graphics card is not used to fullest, what if they have run out of graphic card's memory and system memory is used, it would slow down performance really a lot and GPU + CPU wait for memory transfers.
There is at least something to think about.
At old days there was agp aperture setting in bios, when that was set to smallest possible setting performance of games usually got better, I have been away from computer tech world too long to know if there is equivalent today, but I know there is usually reason why computer is not under full load, something is causing waiting.