That's the point about doom 3, this game will be using features of direct x 9.
You can still run doom 3 with a decent card, but this game was supposed to be using the full features of direct x 9.
So it's basically your choice, as right now doom 3's leak had to have a high end system to run and my geforce 4 ti 4600 could barely get away with fluid motion. Tho the game wasn't optimised and will run much better when you see it as beta(which the e3 leak was in alpha), or a demo or the final release.