What lens has a nice bokeh and zoom? (for nikon d5k)?

My budget is around $400…

You could look at tamron's and sigma's 17-50 or 18-50 f2.8

Bokeh is a result of many things - but one of the key is the number of blades in the aperture ring - so, ideally look for 7 or more preferably 9 or more blades and you will get a creamier bokeh - the manufacturer's list this on their websites…

I have found Sigma to be some of the best for this… Though some of the more recent Nikon pro lenses are very good - the 24 - 70 or 17-55 Nikons are both good.

Lenses which are f2.8 completely through their range are generally pro lenses and while more expensive tend to be be better for this.