Carmack and his cronies don't program on dink PC's 😛
Not sure what they would be using card wise. If you pic up the E3 video they show Carmack at his desk working and his monitor kicks ass. Looks like a 24" 16:9. Someone else may know what they were using card wise. I'm sure they were running on reference GF3 cards though. Doom3 was the program they used at MacWorld to demonstrate it's capabilities.
On a side note, at E3 they said the engine is completely finished (well not counting futher optimizations until release). They are working on the actual content now. w00t, the hard part's over.
Edit: I mean that they had GF3 cards to actually test the game with, not program on. Just guesses here btw.