portal could be made on n64..the graphics aren't that special. I've dealt extensively with the source engine. In portal most of the levels are very very clinical looking, they don't vary much until you get to the gritty end, and then at that point they aren't much of a step away from resident evil 4. If i really wanted to be a dick I'd port a portal level to half-life.
The key is to port the game, not the graphics. The hardest thing to port would be the textures, but gamecube/wii can do phenomenal textures.
Also, **** metroid prime corruption looks better than portal. just throwing that out there.