5D2 will be mind blowing IQ wise (compared to your 20D). I will be better at AF tracking than your 20D. The only thing you will REALLY notice is your reach with your lenses. BIG TIME difference! If you have long lenses on all the time I'd think twice about this body.
7D (since others brought it up), super fast, way more features......that (IMO) probably aren't used, or confuse 60% of the owners. The grip & layout is very nice, and personally being a prior 1 series person I enjoy some of the features that have transferred over into this body. IQ wise, the story's you are constantly reading on here would make you think it's better than sliced bread. After shooting about 200 shots with mine now I will honestly say I'm not that impressed. If I had not shot with a 5D2 then I probably would be a bit more excited about this body. But really, I don't see THAT much change from the 40D I sold to get it. However, I will say I haven't shot sports with it yet.......which is the main reason I bought it.
Bottom line, if you can work with probably 20% better AF than your 20D, and don't need the 1.6 reach I wouldn't think twice about the 5D2. If you are looking for more speed& the same reach then you're all over the 7D.........just don't shot a 5D2 after you buy it.....lol