In my bios I see AGP aperture size under video settings, it is set at 64 MB .
Can anyone explain what is its use and what is best to set in it ?

No one answered yet ?

The AGP aperture size (measured in MB) is memory that has been reserved from your system RAM for graphical processing. This takes the RAM out of use for you and your Operating System. I see this as a common problem with people that know they have a certain size module(s) of RAM and then look in a program to find they have 8mb, 16mb, 32mb, 64mb, 128mb, and 256mb less memory. This setting was needed back in the day because GPUs did not have as much memory as they do today. This was to add to the memory. In many pre-built systems there is integrated graphics, this is similar to a graphics card but much less powerful. (Not even a fair comparison, as we all know).

Now, how much should you dedicate?
If you have a video card with 256MB of memory usually people recommend that you put the size to 64MB or the lowest. If you do very intensive work with say photo editing, movie editing or rendering (I'm not sure). I suppose in the near future or even now with some settings on certain games it may perform better with a little extra memory (64mb-128mb).
For cards with 128MB about 64mb-128mb is usually recommended. For cards with less... I would recommend the same amount as 128 as the card most likely cant utilize all of that if it is that old. That was assuming you have at least 512mb of memory.
I would not recommend 256 to anyone unless they're main use of the machine were photo editing/movie editing/rendering and something else I'm sure I forgot.
If you have less then 512mb I would recommend 32mb size =)

People argue that setting this to any setting doesn't really make a difference and some believe other wise. I would take into account the suggested sizes and try them and bench them all. With more memory dedicated to graphical processing you may achieve better game play but other programs **could** not necessarily have to, suffer from this because your taking ram away you and the Operating system to use.
Making this small give you and the Operating system more ram to use. But cuts down on the graphical processing ram.





here is the link for further information.
http://www.techpowerup.com/articles/...ing/vidcard/43

http://forums.tweakguides.com/showthread.php?t=763