Yes, I agree with wysiwyg, a fast lens would be handy if you're looking to get night shots without using a flash. You can buy one of the finest lenses Nikkor makes for around $200 - the 50mm f/1.8G lens. It's sharp as a knife and crazy cheap. The other possibility is the 35mm f/1.8G DX lens that is also around $200. There's very similar and either one would suit your needs very well.
As for your question, yes, I meant A for aperture mode. You will need to use A mode to open your lens to its widest setting. You can test out your current lens tonight and see how well it performs and then decide whether or not to get a faster lens.