As per the Release Notes: http://www.cisco.com/univercd/cc/td/doc/product/ismg/policy/ver23i/reln233i.htm, both the server and the client support Microsoft Internet Explorer 5.5 only.
I can't say I have ever tried configuring VPN between a Cisco box and a Nortel box, but I can't see that it would be too much of a problem, so long as they both ahere to RFC standards. After all, protocols and standards are written and complied with...
Upgrading should not be a problem. I would still back up the ACS database before upgrading - you never know what "might" happen - but it is generally safe to upgrade.