I don't know about adding memory in the way you described, but there's Amiga users out there who have added more than 1GB of memory to their Amiga's through various combinations of other cards. I remember seeing a screenshot of one a while back, the number across the top of the Workbench screen looked so long, lol.
In any case, you're certainly not limited to 128MB, if you have the right budget ($$$).
