I still have my 20 D and I daily use it at my office. The 20 D is still a great camera, the proof is that the 30 D has the same sensor and same digic 2 chip ( a little faster it seems). So canon think that IQ wise, the 20 D was perfect and it's true.
However the 5D add the joy of using a FF camera. On this camera you re-discover all your lens. My 24-70 is no more a 38-112, and my 17-40 is really wide at 17. The cons it that for tele shot your 300 mm is no more a 480 mm.
IQ wise the 5D is second to none except the 1dsmark2 (and only if you have good post processing skills), and is the king of low light condition shots.
Here is one at 1600 isos. The fans of saxo will recocnize Johnny Griffins aka the Little giant :
With a 5D you can make nice A3 prints at 1600 isos. At other settings you can even make nice 50 per 75 m posters. The bokey are very smooth also, but if you print A4 it's a waste of money.