Further testing with a 64 bit Karmic system now shows this problem.
With ramdisks mounted on /tmp and /var/cache/apt/archives (with
a directory "partial" created), the no space message appeared for
a 40M download for both the ...archives directory (possibly valid, but
I thought the default was 64M), and the /tmp directory, with the
same 5.234M additional space needed. Removing the ...archives
ramdisk removed the associated error message, but the /tmp message
presisted, just like on the 32 bit Karmic system. Removing the /tmp
ramdisk allowed the update to proceed normally. All updates
were attempted via the "System/Administration/Update Manager"