AGP Aperture.. What is it and where should it be set?

Question

While messing with my new MSI video card today I came upon a good question.. what should my AGP Aperture be set at for best performance?  Is it based on amount of system memory?  Video card memory?
To refresh your memory I have:
AMD XP 1600 Processor
512 DDR RAM (2x256)
MSI Geforce4 ti4200 64MB 8X
Asus A7A266 Motherboard
30 GB Hard drive
Thanks!

Answer 1

(long post warning)
You've all got a pretty good understanding.
VGA memory on your card is used as required by the application FIRST (for textures, buffers, code, etc.).  Key point - VGA memory is FAST - remember, in the case of a G4Ti4600AGP it's clocked at ~650MHz(DDR). (and in the case of the GeForce 5600FX, that's now 1GHz DDR II)...
AGP Aperature memory is used NEXT (for textures) if your VGA memory is full and the application requires more.  The amount of memory available is as you set in the AGP Aperature setting in the BIOS.
The key point about AGP Aperature memory is that it is the portion of your _SYSTEM MEMORY_ that _can_ be allocated to be used as AGP memory IF required.
What's special about it, is that any access made to it (by the Video card) is done directly through the AGP bus without need for translation (i.e. without CPU intervention) - this makes using the aperature memory faster than using normal system memory. (but still much slower than the local VGA memory as seen above - eg. a P4 can use 400MHz DDR RAM - much slower than the VGA memory not even accounting for latency, AGP bus speed, etc.).
Finally, if after this your AGP Aperature memory becomes full, then the application must resort to using regular system memory - requiring CPU/Memory Controller intervention - and share with other components (PCI bus) also requiring access to the system memory.  MUCH slower.
Setting a large AGP Aperature won't hurt you as it's only going to be used if your VGA memory is full - however, once used, it's taking away from your system memory.  So, if you set your Aperature to 128MB and you only have a total of 128MB system memory installed....
If your VGA card has alot of memory (64MB or 128MB by today's requirements), then it's likely you won't need a large AGP Aperature as the VGA memory will handle the load.  Still, suggested requirements is to set your AGP Aperature at either 64MB or 128MB (and if you've got the system memory to spare, than all the more power to you).  If OTOH, you've got a small amount of system memory, you may want to remain conservative here - but it will still also depend upon the amount of VGA memory you've got.
Eg: in 3DMark 2000, there was the 64MB texture test - anyone running a 16MB VGA card with an AGP Aperature of 32MB could not run this test - even setting the Aperature to 64MB didn't necessarily mean you could run the test due to overhead - from the link I posted above:
Quote
AGP aperture must be more than twice the size of the desired AGP memory space. So, if you want 15MB of AGP memory for texture storage purposes, then the AGP aperture has to be at least 42MB in size.
The good news is that games today don't yet require this massive amount of memory, thanks in part to texture compression techniques, etc. - but who knows what the future holds (hence we're seeing cards with 128MB and now 256MB on board).  A few more years down the line, and the technology will probably change altogether (I've already heard of the AGP bus being replaced).
(whew...I hope everyone understands better.  This is at least my understanding of AGP Aperature - it might not be 100% accurate)