If you have the kit lens, why not try shooting with it first?
If you take a lot of indoor shots, perhaps a flash is a better upgrade. If you take a lot of outdoor scenery, a flash is pretty useless, isn't it? So it all depends.
Forget about the term "cheap lens", you have chosen DSLR.
There are different kinds of night shots. Either you use existing light and long term exposure (that requires a tripod) or you try to shot with additional light for the foreground (flash). Both can be done using your kit lens.
Haha! As you can see, many varied responses. So it really boils down to what you want to shoot, and what you shoot often. If you don't feel limited right now, don't get either first. When you feel limited then you can decide what to get next.
if you like to shoot in ambient light, outdoors, definitely lens.
but if u prefer indoor shots, even if you think you like ambient light, a proper flash is far more useful. since you are asking this question here, i don't think you have tried bounced flash before (i don't mean to be patronising, so sorry if i come across as so).
use a flash with a head that can rotate and tilt, and u are in business. your kit lens is very good and sharp liao :thumbsup:
my recommendation is SB 600. don't go for lens first. maybe the next thing after u get the flash is 35mm AFS DX from nikon :thumbsup:
i'm pretty serious about your d700 reducing the need for flash. think abt it, a d40 with flash becomes more clumsy than a d700 without.
i'd sure like one, but i think by the time i willing to spend on FF camera, the successor of d700 will be out liao. :embrass:
btw, SB400 seems quite decent, but i would recommend SB600 imho. can fire sideways while sb400 can only bounce upwards. i'm using sb800 but i think the 600 is good enuff. the next step up is sb900 but rather ex. see what TS is comfortable with, budget-wise.
TS: suggest u bring ur gear n try out the 400 n 600. then see what suits ur needs. assuming u intend to buy it new.