Many things are quite far up in the air at this point. Reasonable guesses include a CPU speed bump, new chipset (NVIDIA), and new GPUs (NVIDIA 9-series). A rumor not long ago claims that Apple may be releasing quad-core iMacs at MWSF.
No.
Keep in mind that if you choose to wait until January for a new model, you may end up waiting longer. Besides the above rumor, there are no rumors on a MWSF update, or for that matter, any date. So it may be that the update will come in Feb/March.