Is there any difference between having 4 .5GB memcache servers running or one 2GB instance?
Does running multiple instances offer any benifits?
I would also add that theoretically, in case of several machines, it might save you some performance, as if you have a lot of frontends doing a lot of heavy reads, it's much better to split them into different machines: you know, network capabilities and processing power of one machine can become an upper bound for you.
This advantage is highly dependent on memcache utilization, however (sometimes it might be ways faster to fetch everything from one machine).