
Since many cards come with 128MB gfx RAM, 64MB Aperture Size (AS) will be more than enough. Setting a size larger than this really has no benefits (although some do swear that huge Aperture sizes somehow yield enhanced 3D perf). Essentially the AGP Aperture Size is the MAXIMUM amount of system RAM which can be allocated to the gfx card for use when its own RAM is not enough.
The size you set is the maximum, so if the extra RAM isn't needed by the gfx card then none will be allocated, hence setting a large size should never be problematic, esp on more recent mobos. As said the size you set is the maximum system RAM the gfx card can use should the need arise, but the amount the card actually gets is less than half the size you set due to tables, virtual addressing etc. When set to 64MB this will allow the gfx card to directly use UP TO 64MB of system RAM but this will only add an extra 26MB to the gfx card, similarly 128MB uses UP TO 128MB of your system RAM but 128MB of system RAM only provides the gfx card with 58MB of extra texture storage.

So basicly 64MB AS is fine for cards with 128MB, although 128MB AS will do no harm and is probably the best setting for most people to use providing they have at least 256MB of system RAM and it doesn't cause any wierd problems. Setting it higher should do no harm at all providing it is no more than half your system RAM, but it is pretty pointless and some mobos or component combinations can become problematic when the AS is set higher than 128MB, or even 64MB sometimes.