D90 or invest in new lenses for my D40?

I have a D40 with the 18-55mm kit lens, a 55-200mm VR, and a SB-400. I'm not happy with the flashless low light shots I get in dimly lit rooms, night time carnivals, etc. I'm not sure if I should improve my lens collection or upgrade to the D90. I still have a lot to learn on the D40 and don't know whether better lenses or a better sensor would satisfy my needs the best. I have a window of opportunity to get a moderately discounted D90 kit (10-15% off), so I'm in a bit of a crunch to make a decision.