64MB vs. 128MB?
Well most 128MB boards (AGP 4X) had their memory clocked at 444Mhz. The 64MB's were clocked at 500Mhz.
All AGP 8X boards had 500Mhz memory clocks regardless of their memory size.
Even if you do get a board clocked at 444Mhz, the memory is at least rated 4ns = 250 Mhz x 2 = 500 Mhz DDR.
That and many of the manufacturers ship faster memory with these boards anyway since they can use them for their 64MB boards as well.
So your 128MB board may have memory that's capable of over 64MB stock speeds.
And the memory makes a difference, if only in those games that have large (uncompressed, or even VERY large compressed [Doom3?]) textures. See the Jedi Knight II High Detail benches below.
http://www.anandtech.com/video/showdoc.html?i=1643&p=4
In some cases, it's a round a 33% gain.