Which 18-200mm lens for a Nikon D40x?

I'm currently doing A level photography, and i have a Nikon D40x (not my choice. Would have gone for something better)
i have both a 18-55mm lens and a 55-200mm lens
but i find it annoying having to swap lenses, i was wondering by how much the quality decreses if i were to buy a Tamron lens not a nikkor as being a student i don't really have the money for a £450-£500 nikkor one.
i was also wondering if it actually is worth buying a 18-200mm lens when i have this range over 2 lenses?

I've been using the Nikkor 18-200mmG VR for about three years more or less, on a D40, D80, D1x, and for the past year, a D200. Best thing since sliced bread. I wouldn't stick a third-party lens on my D200 at gunpoint. And yes, it *is* worth not having to swap out lenses.

You might like this one

Nikon 18-200mm F3.5-5.6G IF-ED AF-S VR II DX Lens

DX format 11.1x zoom lens with 18 to 200mm focal range (35mm equivalent: 27-300mm)
Vibration Reduction II (VR II) stabilization system with Normal and Active modes
M/A mode for rapid switching between Autofocus and Manual focus operation
2 ED glass and 3 aspherical lens elements ensure high resolution and superior contrast
SWM (Silent Wave Motor) for fast, whisper-quiet autofocus

I'd say it all depends, if picture quality is not as important as convenience then yes, get the tamron. If you want to find a compromise go for nikon's 18-105, load of range, good quality and quite affordable. Oh, and actually using that shift key on your keyboard makes your post readable and makes you look less like you're intellectually challenged.