I don't think a lens will fix your light issue if you're shooting wide open with a f/1.8. You can only get another lens that will open up to f/1.4 or 1.2 from there and that won't give you THAT much more light. It wouldn't be worth the cost IMO, cause those lenses are quite spendy.
Consider going outside, getting near a window, or upping your ISO (just know there you introduce more noise, and the D300 is pretty noisy above iso 800).