Calibration isnt going to do a damn thing to a TN panel LCD. They simply cannot handle dark colors and the color shifting will be a big hassle since theres no way to tell which color is the "real" color without another monitor to use as reference, its especially difficult when working gradients since youll have to deal with the "fake" gradient the monitor is giving. I have an IPS panel at home which has practically perfectly even color, Ive noticed numerous mistakes when taking my school work done on the 20" imac to my home computer, like gradients I added which I thought were strong enough on the imac not even being noticable on my home monitor since I was fooled by the color shift.
TN panels also dont display as many colors since they are 6bit instead of 8bit. TN panels that claim to be 8bit are only interpolating extra colors through tricks, like an upscaled dvd vs bluray.
more info:
http://www.tomshardware.com/2007/11/27/apple_imac_display_problems_reported/
I guarantee that if you get a high quality monitor and then come across a 20" alum imac that youll be thankful you went with the good monitor. There wasnt a day that went buy I didnt get pissed at that piece of crap screen since I was so used to working with a good one. To really understand the difference between them you need to use both, people who have been using TN their whole life will say that the 20" imac has a great screen for design, but its completely false and its never more apparent until you look at your work with a good monitor.
The Mac Mini is
not going to be slower in Photoshop/Illustrator/Indesign/Quark/etc than the iMac if you get 2gbs of ram. 2D programs arent affected by processor speed except when running filters, they only consume large amounts of ram because file sizes can be so big.