as the title says, me and a friend are both using the GOG version of BG2, and when we go to start the game, our UI and gamespace both appear black.

we are both using custom portraits if that makes any difference.

he is using XP, i am using 7

Have you tried using windowed mode?
If it works, you can then press Alt/Enter in game to go back to full screen.

fullscreen just crashes before i even reach the lobby, windowed mode does the blackscreen thing.

do i need that fixpack to play over gameranger?

I don't play multiplayer, or know anything about Gameranger I'm afraid.

http://www.gibberlings3.net/bg2fixpack/docs_core.php
But this is a list of core fixes in Fixpack, a quick read will show why most would consider it a must have.