I think this one might be worth getting printed up.
I agree, it's a fascinating image! I think, though, that you ought to lose most of the colour, but slightly enhance the yellows and reds.
Here's an alternative interpretation:
I used Photoshop Elements and a Hue/Saturation layer, where I adjusted the saturation in the following channels:
It's been a while since I used Aperture, so I can't recall whether it was possible to do selective colour adjustment like this, but I would be surprised if it wasn't.
Bonus: this may
eliminate your purple fringing problem, since it will now be all monochrome. You may need to adjust the luminance of the magentas and blues to make it work well at full enlargement.
From a D200, how big an enlargement is reasonable?
That depends on the intended viewing distance.
I just printed an image taken with my 20D at 200 PPI to 30x40 cm, framed it behind glass, with a 29x39/40x50 matte. It looks very nice, and since it's on a wall, people are not likely to bring a loupe to it to see if any detail's missing.
Seriously, though; it is
all about how your image will be presented.
I suspect that the subject matter is good for resizing with Genuine Fractals to a fairly large print size.
If you're unsure, try a few different sizes, and decide for yourself. If price is a concern, think of it this way: the prints that were the "wrong size" may still be keepers, and perhaps you can make a nice gift or two to family, friends or a neighbour.
I'm skipping the rest of your questions, since I've just about answered what I think I can.
Good luck with your printing, I hope you'll be satisfied!