i'm not really sure why you're getting such a low score, but i can answer your second question. video cards, every single one i've tried, that needs a separate power lead all work fine booting into windows etc, maybe its because of the lower clock speeds that they run for 2d. but during 3d things, a 5900xt i was borrowing was running about 2x worse than the 4200Ti i'd been using previously, simply because i'd forgotten about the extra power lead i needed to plug in. if you've installed the nvidia drivers, they would have picked up on the fact that the extra power lead isn't plugged in, so i guess thats not a reason for your low score (not saying you did, but stranger things have happened).
in short: yes, underpowered hardware will work, but it will perform more slowly than if you were running the correct amount of power through it.
generic questions about running 3dmarks:
-- Are you sure all the settings in 3dmark are default?
-- Have you checked to see that antialiasing or anisotropic filtering havent been enabled through the nvidia global driver settings?