I had all the same concerns when I bought my 24" / 2.8GHz / 2GB / 500GB about nine months ago. ALL UNFOUNDED!!! The big 24" screen is PERFECT, it sits on a very narrow desk, my eyes are maybe 14" away from the screen and again it is PERFECT! I couldn't imagine going down to the 20" from this. The standard 2GB RAM is likewise PERFECT for my needs, and I also multitask (including FCE). The 500GB drive is completely adequate for my forseeable needs; it is currently 1/3 full although I do have apx 600GB of data safe on a 1TB external HD connected via FW800 (the only way to go!)
I was also worried about screen brightness, screen tinting, dead pixels, fan noise, the flat keyboard, the Mighty Mouse, and everything else negative I'd read on these forums in the last five years. ALL UNFOUNDED. My advice -- don't worry about it, don't worry about them, buy the best iMac you can afford, and love it! This is (IMHO) absolutely the best home computer ever designed and I feel this unbelievable pride of ownership whenever I use it. Soon you'll know what I mean.