Possible
I'd say it just depends on how close you're getting to 64MB ... a pot of boiling water doesn't have to be full before water starts to splash out ... like i said, probably anything over 40MB.
i finaly got some professional talk on this here:
Fragmentation can also refer to RAM that has small, unused holes scattered throughout it. This is called external fragmentation. With modern operating systems that use a paging scheme, a more common type of RAM fragmentation is internal fragmentation. This occurs when memory is allocated in frames and the frame size is larger than the amount of memory requested.
now if you look at my previous posts and compair what i've said to this, then you will agree. i said page sizes get smaller and bigger and space out with little space in between ... if you're utilizing the GPU to look for big enough spage and it can't find it, it will either issue pages with locations at the end to find the rest of the page in the ram ... (thus fragmenting) ... then pages start to break up into several peices as things get worse. or it starts using the long "bus" ride to the system ram which will obviously cause some slow-down and then you're not even utilizing the GPU at all. with this being said, any higher resolution or request for more textures (especially on newer games) will show a greater difference between the 64mb and 128mb regardless as to whether or not it ever uses more than 64mb.