Well... Depends a lot on when you will be here. I went out this evening with a pair of camo jeans and a long sleeved cotton shirt. Sunday morning I had on Under Armour Cold Gear tops/bottoms, polar weight fleece top/bottom, pants and shirt, with a jacket. If it gets any colder I will be wearing my insulated bibs and parka.
I guess what I'm saying is that you never know what the weather will do in Illinois. I'd say plan on it being on the cold side. It always seems to be humid where I'm at, and that makes it seem that much colder.
Good luck, and leave a couple nice ones out there for me
rw