I suppose the first thing you need to consider before splashing the cash is whether or not you can get more light on the situation which will be a cheap way of bringing noise levels down by using lower iso. Maybe higher wattage light bulbs or strategically placed lamps.
If you do want another lens, what focal length are you thinking of? I'm guessing from what you say that you're currently using the 18-55 kit lens at it's widest end going by the fact you're using a f/3.5 aperture. Is it a 50mm lens you're thinking of? It could be quite tight indoors. Maybe a 35mm or even 28mm would be better? You can check this by using your current zoom at different focal lengths and seeing what works best for you.
You don't necessarily need a 1.4 lens, a 1.8 would be nearly as effective in terms of getting extra light in and hence requiring a lower iso (less noise) but would generally cost a lot less. Nikon's 1.4 lenses are generally a lot of money, the exception being the 50mm 1.4 but that is still twice the price of the 50mm 1.8 equivalent. Maybe also consider the 35mm 1.8g and 28mm 1.8g.
Do bear in mind though that if you use any lens at an aperture this wide, your depth of field is going to be very shallow and so your focus is going to need to be spot on. It's a nice effect when done right and really separates DSLR's from cheap camcorders but it's not easy to nail.
