hi,
we have got a 'modular bim server' AC13&14 running on a mac mini which works rather well, at least for the AC14 projects. luckily the AC13 projects are legacy anyway and will be archived soon.
regarding your question though, here my 2 cents worth on determining factors for teamwork setup (btw. i was trying to get some other people to comment on a similar issue regarding hardware a while back, available here:
http://archicad-talk.graphisoft.com/viewtopic.php?t=35906&highlight=😞
1. project / file size & number of users > basically & simplistically put one of the limiting factors on any server is read/write speed, i.e. a 7200RPM harddrive can only do so much;
2. hardware cost > what gives you the best bang for your buck;
also check out the following posts:
on archicad user management:
http://goo.gl/Eyozv
article on archicadwiki on best AC15 TW practices:
http://goo.gl/zliik
at the moment in mac environment, i reckon the following setup is the best:
mac mini server, 2.66 GHz, 8GB RAM, 2x500GB
reformat & install with normal OSX, then install bim server and the projects on the two separate drives for optimal access speed.
but really i would wait until the next gen macs come out and include thunderbolt - also note that the mac pro series is rumoured to be upgraded soon...
any other thoughts are much appreciated.
ds.