I think 64 MiB of data storage (disk) should be enough - but I don't know how many data uses each User Manager's user account in database.
But there is other thing: I don't know why, but RB1200 that I've bought in July has 1 GiB of RAM. Actually it seems that dealers are selling 512 MiB version and so do you have on website. I think 512 MiB is plenty of memory and my question is - how could somebody use it?
My RB1200 has cca 970 MiB of free memory
I've increased DNS cache a little but it will not use a lot. Maybe the memory could be used as proxy cache in some way? AFAIK it can use disk storage only. Maybe very big (old) log entries of User Manager may need it?