The Senate race is pretty cut and dried since an Allen victory would almost guarantee that the GOP holds the Senate.
The race we have that could dictate Congressional outcomes is the 2nd district. A 58% Bush district and won by Drake with 55% in 2004; the numbers here could give us some insight into what will happen the rest of the night.
First and foremost, I think if Drake loses it is likely the GOP will lose the bulk of the competitive races. Her losing, even against a great candidate, would probably signal a "wave".
Predicting the mrgins if she wins is much tougher. The latest poll shows her with a 51-43 lead, but another recent poll actually has her trailing. This would indicate she will have a hard time getting more than her previous 55%, and maybe a hard time even getting to 52%.
I think if she can break 55%, it will be a major signal that things may go much better for the GOP than expected. I can't promise they will hold the majority if she breaks that number, but the democratic margins will probably be low. For the purposes of this analysis, we will assume if she gets 55% the GOP will lose 15 seats giving the dems 218. For every % point Drake is under 55, I think the dems probably gain 3-5 additional seats. So for instance, if Drake gets 52 then the dems will have 227-233 seats.
Obviously this is not an exact science by any stretch, but I think we will probably be able to pretty accurately tell how the election will go by Drake's margin.