Hold up... Shadow Warrior.
Duke Nukem' 3D engine but with way better auto taunts and swords! It should be able to run on crummy hardware well, I think a P2 450MHz and a GeForceMX were the king s*** back when it was released. I think 3D realms has a full download at their site, they even released the expansion a few months ago that they never released when they still sold the game.