why does she want to upgrade? What is lacking in the D50 for her? Identify what that is and buy the best body you can afford that provides it. If it is just a case of wanting a new camera that is a different story.
The old adage is - if you want to invest, invest in glass until the body lets you down, then upgrade the body...
If your wife's 18-55mm lens is a Nikkor, then it will definitely autofocus normally on either D90 or D5100 (or and other Nikon DSLR).
I imagine the Tamron is a few years old, so it's unlikely to have its own AF motor - only more recently has Tamron been making such lenses. If I'm right, it would not autofocus on a D5100, but would on a D90.
Thanks for the replies folks. I suppose a large part of it is she wants a new camera but she also wants more resolution. She will have access to my lenses so the resolution increase should be noticeable when used with these.
She has now grabbed a D90, new and at a bargain price of £449! Sounds good to me.
Ahh, so you are asking about upgrdes from the Tamron; not the kit lens that came with the body. I'm just wondering if it be worthwhile thinking about upgrading to a ''pro"-Nikon lens compared to the kit lenses. I have the 28-80 and a 70-300 kit lenses. They seem o.k.
I was really wondering if the Tamron would work with the D5100 but I know now that it wont. Therefore the most sensible would be to the D90.
I'm sure the Tamron is fine for smaller prints but what I meant by referring to resolution was that if she was going to stick with the Tamron and that was all she had in that focal length it would indeed be more sensible to upgrade the lens first.
As she has access to better glass then she will be able to take advantage of the better resolution in the D90 from the D50