I use the Sigma 30 mm f/1.4 on my D40x for low-light parties. It has the advantage of being an HSM lens - Sigma's equivalent of AF-S.
I also use the 50 mm f/1.2 and 35 mm f/1.4 manual focus lenses for when there is not enough light for autofocus - manual focus lenses tend to be easier to use than autofocus lenses in manual. I have a modified K3 screen in my D40x for this purpose.
Best,
Helen