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I finally solved setting up multiplayer BG2 (BGT in my particular situation) on two wireless Windows 7 laptops. This can be done without resorting to Hamachi or GameRanger.

1) Turn off gamespy in the baldur.ini file.
[GameSpy]
Enabled=0
You might also need to do this in the Config miscellaneous page.

2) Run BG2 in windowed mode. This means you will have to turn off the widescreen mod or run it at less than your full resolution.

3) Allow BG2 through your firewall on both machines.

Note that totally turning off my firewall caused multiplayer to STOP working.

4) Turn off uPnP on your router

Something about it prevents BG2 from networking.

I did not have to do any port forwarding (and in fact, if both laptops are on the same router, this will likely cause additional problems).

On the other hand, if you were playing someone over the internet, you'd probably need to turn on port forwarding. ports 2300-2400,6073 for both TCP and UDP is what seems to be recommended.

I also increased my cache in the Config miscellaneous page from 300 to 1000. I don't know if it's necessary, but it was one of the first suggestions I tried, and it doesn't seem like it'll hurt anything.
I never saw the appeal of BG multiplayer but kudos to you for doing this post for anyone interested.