I should make it clear that I am talking about the mac that is only just shipping with "Intel core duo"
http://www.apple.com/macbookpro/
not historic ones like mine except that the new one is in the same box, an industrial design classic and inter alia using to within 7mm of the edge of the top for the 1280x800 screen.

I am not sure whether I would quite go so far as describe using GraphicConverter as joy and not sure what the universal has in it; what you would have to do is go to a mac dealer and get the trial download http://www.lemkesoft.com/en/graphdownload.htm
onto a demo box and try your RAW files.

You should not directly compare processor speeds, as the OS is far craftier. GC has a good record of keeping up to date with Mac OS and I think Nikon will follow. Less confident about Adobe as their so-called MacOSX progs are hopelessly memory hungry and cumbersome compared to true natives; seem to have the MS habit of adding more and more superstructure and never going back to the foundations. There would be a lot in the editing field that you would still need to go back to PS for (where I use 7 year old Color-it!!).

I find slight difficulty comparing mac with Wintel as no-one whose Wintel system I have had the misfortune to try and use in emergency seems to know how to use it themselves, never mind explain to me. I also have to admit that I did have to use a paper manual to change from Mac OS9 to 10, unprecedented since Excel 2...and I would have to say 3 year extended warranty was essential having had new hard drive and superdrive under mine at 3 years minus 10 minutes or so.

But I am sure you would find it worth spending an hour with a dealer and their prices are no worse than internet ones.