Well I know the books say put it to the amount of graphics card RAM you have, but EHHH, I don't think much more than 128mb is necessary simply because your system is trying to use the RAM, then you're trying to suck out RAM for your graphics card to use, sooo too much of it = no good.
Also, dale, I'm not 100% positive on this, but I've heard like..128mb might be the best setting compared to 256mb of 64mb regardless if obviously your card has enough RAM....even if i had 2gb of RAM and a 512mb card I'd probably still have it at 128mb, although yes I believe 64mb would be fine also..
I don't think anyones noticed any actual performance gains or losses from changing the aperture