In several ways the 12mp compact could still be less quality than a 6mp Digital-SLR.
Due to the physical size of the sensor and the lens the compact has a tendency to have chromatic aberrations and purple fringes.
These aberrations look like red, blue, purple and sometimes green edges outlining some objects in a photo.
These are more noticeable when the photo is enlarged or zoomed in on using a photo editing program.
Also images above 200iso on a compact camera compared to a Digital-SLR are grainier(noisier).

Not sure what product you are asking about.
There is a Nikon D5000 that is a Digital-SLR. There is a Nikon Coolpix 5000 that is a 5 megapixel compact camera. There is a Nikon 5000DX module for promaster flash units.

No matter what Digital-SLR you get they all have a full auto setting making them a Point-and-Shoot camera until you get the hang of full manual or semi full modes.
Action shots in any sports are easier to get when the camera has a fast burst mode (sequential shooting).
Entry level cameras are usually at 3-4fps (frames per second) while semi-pro and pro are 6-8fps and higher.
Another factor that you may want to consider is physical size. I shoot with a Sony a700. It is a pretty big camera and my wife doesn't care for the size. She uses my much smaller a200.
The Canon XSi is physically smaller than a 50D.
Try several cameras at the demo area of your nearest BestBuy and look for a camera that feels good in your hands.

Depending on the megapixel count of your camera, the resolution setting and the type of file the image is being saved as, 18 megs can be eaten up very quickly.
A 10mp image at full resolution and using jpg compression could be about 2-4 megs big.
Much higher is saved as a RAW file.
Internal memory is really just a safety buffer if the main memory is full.