From: Essex, England
When you say you have opened up port 2000, can you confirm you have the following protocol definitions (and then a protocol rule which uses these definitions):
Protocol, Port, Direction
UDP , 2000, Send
UDP , 2000, Send Recieve
TCP , 2000, Outbound
If that doesnt work, try also enablind outbound and inbound secondary connections for each definitions (i.e, Port 1024-65535 Outbound and 1024-65535 inbound). Thats a bit over the top, so if that works you should then tighten down security (i would think it may just need secondary inbound connections to port 2000, not all ports).
If you still have no luck, just create a "All IP Traffic" protocol rule and see if it works. If still no luck, try also using the firewall client.
Also, check the firewall and IP packet filter logs (located under your ISA Server\ISALogs directory). You can use bt-Patrol from http://www.burstek.com/ptform.htm to view the logs in realtime, or just to view them in an easy to read fashion.
If you still cant get it to work, let me know and i'll download the game and give it a go.
Hope that helps.