How does turning down your video memery increase performace?

[ChaosLord40k]

I was reading a month or so ago on transgaming forums, that turning down your video card memory will increase your performace. I've tried that and it realy does work, with a GeForce2 and a Radeon 9600. I play cs at 48-99 and others at a 800x600, and more with 1024x768. I've tuned my radeon memory to around 32 or 16 mbs of memory (still just messing around with values) and just abou every game i've played under winex gets extremely high frame rates (that i can get with winex) but I can get Enemy Territory to cooperate with the memory tuning. It crashes every time I try to start it, when my video memory is low. So exactly how does this imporve your video performace? Is it because it doesn't use as much "something" so it doesn't stress's it out, or what. Im just curiouse cuz I can play a lot of my games (including cs) at a GREAT pace, with my Celeron 2.0Ghz, 512 Mbs of 2100 ram, and Radeon 9600 non pro.
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[jasonlambert]

i would guess it is becuase you have more system ram available in situations where the graphics card shares memory with the system?