To be honest £250 isn't enough to get a zoom lens that would give you noticable quality improvement on what you already have, so I think you would be wasting your money, just to get low light performance or less DoF. Remember that unless you go for the Nikkor AFS or Sigma HSM lenses your body will be driving the A/F motor and it will be noisy and slow. Also think about the future and remember that Nikkon has just entered the Full Frame market so in 5-6 years time there is likely to be a Full Frame Nikon at a reasonable price (sub £1K) and it makes sense to ensure that your main lenses are suitable for Full Frame.
With what you have I would suggest you get a nifty fifty (50mm f1.8) as that will give you a perfect low light performer & ideal portrait lens for about £75 new and put the rest away until you have about £700+ and then go searching for something that's you will notice the difference e.g. Nikkor 17-55 f2.8 DX AFS (digital only), or even looking for a S/H Nikkor 28-70mm f2.8 that's just been replaced by the 24-70mm f2.8 and is suitable for FF.
Alterntatively you can expand your range and get the Sigma 10-20mm super wide angle but that's just outside your ideal price range at about £290