Firstly I have to agree with Harry re image degradation with tubes or bellows. I have a manual focus 55mmMicro Nikkor and even at max extension ( tubes + bellows ) I have not seen any.
As for the question tubes or dedicated macro then I think it depends on what you are aiming at subject wise, and frequency of use.
A dedicated macro lens is corrected for close focussing so will give better results than tubes on your current 50mm. The downside of tubes or bellows is light loss that gets worse the greater the extension. Even though the 105 is a f2.8 lens it will not be f2.8 at max extension for 1:1 reproduction.
As for lens focal length I find the 55mm often does not give as much lens to subject distance as I would like. When I bought this lens I could not afford the 105mm or 200mm and still can't. The 105mm gets good reviews and price point is pitched midway between 60 mm and 200mm.
So a couple of questions for you.
Is the lens only for macro use. If so how important is A/F.
Are you looking at 105 for VR. If so, then its worth a read of Thom Hogans review where he notes that VR performance degrades as you get closer. By the time you are at 1:1 you would probably be on manual focus as well.
If you are looking at close up rather than macro then tubes can be used on any of your lenses. Christine ( Saphire ) has posted some great images using them.
Of course if you have a good bonus
then why not consider the 85mm f2.8 PC Micro Nikkor. Far more versatile than any normal macro lens, or any other lens of similar focal length for that matter.
Don