I have build GNS3 server for multiuser purpose. There is little tutorial about this in this forum:http://www.gns3.net/phpBB/topic314.html
I haven made "stress tests" to my server. Not so long ago we had "bigger" lab running. There were 20 routers (7200) doing BGP together. There was little bit lagging when everything (bgp and stuff) were running. However average CPU utilization for 4 cores was about 30%.
My server has quad-core processor (I don't remember exact model) and 8 GB RAM. You need quite a lot resources if you have 30 x 5 = 150 routers running at the same time and doing something useful. If you do it like I have done using VNC you need to think also using 1 Gb interfaces on your server. Maybe 2 groupped together (Etherchannel).
BTW I have noticed couple of issues with VNC. Just while ago I moved from VNC to FreeNX. It's also remote desktop tool. It seems to be working much better. There isn't so much tuning when installing FreeNX. With VNC I had big issues when sessions timed out etc...
I also want to remind you that GNS3 doesn't support multiuser environment. You have to write scripts to add users etc.
Since I have written tutorial I have tested following things:
- Multiple instances of PEMU is pretty difficult to handle
- Connecting to real world is possible and easy to setup. (Real world can't be virtualized so you have to decide who can use Physical NICs and when)
- FreeNX is much better than VNC.