Do photostock companies accept images that have gone through a noise reduction program?

I've been using 'Neat Image' as a noise reduction software, mainly because I have a D200 and it can have a lot of noise, even with 400 ISO. I've had about 20 images accepted by one of the big stock companies, but none had noise reduction. Would this be a factor in whether they select an image or not--assuming all other things being up to their standards? Or should I wait until I have enough money to buy a newer Nikon (since I have all-Nikon equipment), so the noise factor won't be so important?

Yes but It depends how the final image looks.

It does not matter if it has been through the washing machine. The only thing that matters is the final result. Many times noise reduction programs can soften the photo, and for THAT reason the photo may be rejected, but not for the simple fact that noise reduction, or any other program, was used. An excellent quality final image is what they seek, they do not care how you got there.