This comparison attempts to answer the question 'what benefits can expect to see from upgrading a 5 year old DSLR to a current version with particular emphasis on image quality?'

Nikon D5000

I bought the Nikon D5000 to replace the aging and battle weary Canon 300D.  The 300D has been a warrior,  performing in all weather - rain, hail, ice, -8C and even survived falling 4 feet onto concrete.  The 300D still works perfectly and suffered only a crack to the small settings window - easily shored up with thick transparent packing tape.

I was intending to replace the 300D with the Rebel T1i, but found the budget simply did not stretch that far.  Then a blasphemous thought entered my head - "why not consider something other than an Canon?"  Within 5 minutes I discovered the Nikon D5000.  The main attractions for me were:

The Tilt and Swivel LCD Screen - this is invaluable for Astrophotography when the camera is often attached to the telescope when in the vertical position.  No more crawling under the scope to find focus and check settings.

ISO 3200 and extending (at your own risk) to ISO 6400

The 3200 setting is very well corrected for noise.

This halves telescope tracking times over ISO 1600.

Live view - allowing focus to be done on the large rear LCD Screen without looking through the viewfinder.

High Definition (720p) movie shooting.  This may make the camera useful as a planet camera.

Intervalometer - this allows the camera to do time lapse photography.  I'm really looking forward to shooting wide vistas of the starry sky passing overhead.  An outstanding feature I've already employed on a little video called 'Astro-Cafe' - see it here: http://www.youtube.com/user/Coastastronomy

The image sensor is 12.2 Megapixel - almost double the 300D's 6.3 Mp sensor area.

In-Camera Retouching

It's a fact that once you download your images if you don't edit them there and then, you probably never will.  Being able to check and improve an image in-camera is invaluable.

Long Exposure Noise Reduction

Although it is arguably more effective to do this with Software, that only applies if you remember to take dark frames in the first place!  I will have this switched on for the first half dozen frames of each session.

High ISO Noise Reduction

I was dubious that this could be effective - but the side by side comparisons with the 300D show it to be very effective.

Nikon D5000 V Canon 300D

Image Sensor

I wondered how much of a difference 6 years would make on the quality of image coming off the sensor and found if it wasn't for the bigger image produced by the 12.2 MegaPixel sensor you would not be able to tell them apart.  Strictly on image quality there is no difference.  You cannot tell which sensor is which.

Resolution

But the 12.2 Megapixel image does of course have more resolution and simply produces a bigger image requiring less enlargement to print or view.  If you view the Canon image beside the Nikon one, the canon image has to be enlarged to match the image scale even although it was shot at the same focal length (as you can see by the thumb nail images).  

Firmware/Software

Also when at high ISO the Nikon streaks away from the 300D on image quality.  The Nikon uses high ISO noise reduction.  I'm sure the latest Canon's also employ this feature and so It seems that software development has made the biggest impact on image quality improvement rather than sensor development.