Apart from the Sigma mentioned above, its natural competitor is the Tamron 70-300mm F4-5.6 Di LD Macro.
It can be found brand new for around £120, that's in the UK too, and it has the AF motor so AF works on the D40. (It's the D40 that needs the powered lenses isn't it?)
I've owned both lenses in Pentax fit, and they have their strengths and weaknesses. Purple Fringing was evident sometimes in the Tamron, although I never really found it a problem and could be processed out anyway, and the Tamron was arguably a little sharper than the Sigma.
The Sigma is an all round better lens I think though, although it's about £50 more expensive in it's APO guise. The non APO version can be found for £109 though, which is even cheaper than the Tamron, and it still has the AF motor (a non motor version is available for £10 less). Non APO Sigma vs Tamron is pretty equal I think.
I've still got the Sigma, but gave the Tamron away to a mate, so that probably tells you all you need to know about what I thought between the two!
There are plenty of reviews of the two lenses though, as it's a commonly asked question when people are looking for a budget zoom.
