As far as stability is concerned I haven't found any difference
between 32bit and 64bit Ubuntu - with the 64bit one being quite a bit
smoother in operation on the same hardware. Regarding memory, 8Gb is a
total waste on a 32bit OS and that alone would make 64bit a 'must' :-)

Splitting off the /Home partition is a good idea as is making an
external backup of all your data. Taking a 'disaster recovery' image
is also a good idea but I've not had experience of doing that so
others will chime in here no doubt