[Zope] memory size optimization

[lty]

Hi all,
I am wondering if, having say 15 zope sites, with an average zexp size 
of  70Mb (but two of 200+Mb), it is better to have multiple instances or 
not in order to reduce overall memory consumption.
I know memory is cheap, but I really can't upgrade my server, I have 
1.5Gb of ram and I had to tweak zope.conf to reduce cache-size and 
pool-size. My server is still using swap, I have now 3 zope instances 
(mainly because of products versions conflicts) and I don't know if 
using 2 or only one instance would be better memory wise.
Thanks in advance,

Lorenzo