The 20" iMac uses a cheap TN panel which is not recommended as you main screen for photography, only the 24" uses a decent panel. In any case, most discussion on the IQ of iMac screens is pointless, because you can always connect any screen (with DVI or VGA input) to your iMac.
This means, if you are not satisfied with the IQ, you can get a good external lcd and use monitor spanning.
IMO the glossy screen issue is way overrated, especially in the notebook space. Since the 20" iMac also uses a TN panel, what I say now also applies to that machine as well. I have a ProBook with matte display. And regardless of whether glossy screens are supposed to be `worse', the IQ (particularly the gamut and contrast) is nowhere near the Samsung lcd on my desk at work (it uses an MVA or PVA panel, not sure anymore which). The gamut is a lot larger and pictures look significantly different on both monitors.
So mostly, I use my ProBook's screen to pre-edit shots and to have a nice overview over my image library in Aperture.
Regarding the iMac, you can either get the 24" model which has a decent panel and which you can calibrate, if you follow instructions properly, you shouldn't have problems to calibrate it (according to the documentation that came with my Color Spyder 2, all calibration should happen in a dark room, no matter the monitor type).
If you go for the 20" model, then you should get an external screen. Not because the screen is glossy, I think this will have much less of an effect, no, because it uses a TN panel. Do not get a cheap screen, all cheap monitors use TN panels. Monitors with other panel types are markedly more expensive and in the neighborhood of Apple Cinema Displays (that's one reason they are more expensive than competition).
In either case, you don't need to get a Mac Pro.