Well, here's my 2 cents worth.
AGP aperture is the amount of system ram you're willing to let your graphics
card use. It used to be that setting it to 1/2 of your system ram was
considered normal, but I guess things have changed.
When I was running 256mb of system ram, I always set the AGP to 128
(wouldn't want to set it to 256 and use all the system ram). When I just
went to 512 ram, I played with setting at 128 and 256, but there wasn't any
universal difference that I could detect.
Every once in a while when playing CoD, I'd get a momentary slowdown while
the hard drive was being accessed (with stressful graphics settings). This
weekend I played around with 128 and 256 AGP settings again to see if one
performed better than the other in this game. It didn't. On a lark, I tried
setting it to 64 and it seemed to run much smoother. It may not be the same
tonight, but who knows.
This setting seems to go against everything I've learned about how the AGP
aperture works. To me, this setting should have made the game access my hard
drive even more (bypassing system ram), but it didn't. My 3DMark2001SE
scores are the same no matter if it's set at 64, 128 or 256 (I've never had
a motherboard with a 512 setting, so I can't test that).
The moral is, experiment for yourself and if you don't see a difference
don't worry about it.
Gary