I'm probably not the right person to answer your questions, simply because I am a fanatic.  On the other hand I have 97% acceptance out of 460 submissions to SS so being a fanatic is sometimes okay (I believe sharply_done has an even higher acceptance rate, but then he isn't human......).

Firstly, your D50 is a noisy camera.  Technology has moved on since then.  Also, with 6mp you are going to eventually have cropping problems as agencies continue to increase the minimum file size requirements over the years.  Upgrade to a D40X if you have the money; the image quality is a huge improvement too.

Every digital camera produces noise and artifacts in some shape or form.  To minimise this, you MUST get the correct exposure.  As soon as you tweak the levels you will introduce problems.  Learn how to 'shoot to the right' and take a small white card with you to judge exposure by exposing for bright white with the histogram.

Turn all your camera settings down to neutral; that's sharpening, saturation etc.

I never use noise reduction software.  Some people use it very successfully.  The key is to not get any noise in the first place.  If you expose correctly you'll minimise the noise.

I always check every image at 200% magnification.  I look for noise and artifacts in shadows, reds and blues.  In fact I check every inch of every image, but then I'm weird.  If I see any slight hint of anything that the image inspectors might interpret as noise or artifacts, I gently smooth that area using a soft brush and Photoshop's smudge tool set at 2% or sometimes 3%.

I don't use RAW.  I found that by getting everything right in the first place my D200 could produce wonderful images as jpegs.

Watch out for sharpening.  Do it in the PSE file before you finally save as a jpeg 12.  I rarely apply sharpening, but when I do it tends to be very small - around 45, 1, 0 or sometimes 65, 1, 0.  If you need to sharpen more than that you'll just introduce more noise and other 'orrible stuff.

If you've got a great picture and noise etc is a problem, the solution is often to reduce it to a point where the problems disappear; okay you end up with only a small image at the agencies but at least it gets there and can sell.  This is where your 6mp is a limitation.  Upgrade to 10mp if you can afford to do so.

Hope this helps.

... (I believe sharply_done has an even higher acceptance rate, but then he isn't human......).

97% ... that's amazing hatman12!
I've gone on routes with SS when I've had a perfect acceptance rate, but overall I'm "only" in the low nineties.

The only advice I have is to downsize your shots to 4MP, which should help with noise, artifacts, and sharpness.

Good luck with your next SS application, Nazdravie!