I would wait until MWSF, it's only a few weeks away. Even if they don't announce a new iMac, they may lower the price. I bought the 20" the day after Thanksgiving with the $200 off at the Apple Store. I went there expecting to get nothing, I was surprised at the special for that day only. If they had not been running that special, I would have waited until MWSF.
This 20" screen is awesome, you WILL like it!! I'm also a switcher and this is my first MAC. 😀