After further review it seams like your memory usage is appropriate. My Joomla install on a page with Joomla only content alone is 20MB. With CB this increased to 30MBs and with GJ increased to 39MBs (GJ is nearly as large as CB it self) then various other plugins increases to 42MBs. Add in everything else you've installed and it could easily hit 60MBs. I recommend just increasing your memory limit to perhaps 72MB or 82MB to see if issue resolves. If not then you've probably a memory leak coming from one of the extensions.

My standard joomla install pushes 20mb on the rebuild. If I then add community builder it remains the same on a generic page but this then increases to just over 30mb when accessing CB driven pages such as registration. If I then throw in CBsubs, kunena and Group Jive it takes me over the 65mb limit.

CB only loads on CB pages. That's a lot of libraries you're preparing.

Given your experience in joomla/cb/additional extensions do you feel a site will ever exceed 128mb and what do you recomend the max php memory limit to be?

It should never exceed 128mb unless you have a very very large userbase and your extensions aren't optimized.

I am seriously considering changing hosts and need to get my head around this before making a decision...any advice would be most appreciated.

Changing hosts won't change your installs memory demands. You've a lot of large extensions installed. GJ and CBSubs are both about the size of CB it self so expect at least 20mb increase from having all 3. With a base of 20mb that's 40mb already. Add in Kunena that would up it some more and even further so based on category count. Then total in your userbase and it goes up further. Another thing to keep in mind is Joomlas debugger takes A LOT of memory (J1.5s actually crashes my Apache) so once it's disabled expect memory usage to drop by about 5-15mb.

Also note GJ 2.4 isn't as fully optimized as it should be. Next release (GJ 2.5) I've significantly improved PHP performance. GJ 2.4 improved database query performance, but has PHP performance issues that are addressed in GJ 2.5. So with next release of GJ expect memory usage to drop some (5-10mb or so).