REI deal of the day is a cheap bag:
http://www.rei.com/product/767275
they have a big
selection of 3 season bags.
Why a rectangular bag? Much colder for your feet. Also, I've done most of my touring with a down bag - for the warmth, it's lighter & stuffs smaller. And since you are bike touring, not backpacking, if it gets wet by accident, just go to a laundromat or hotel and stick it in the drier on "air".
I've been hailed and snowed on in August in Yellowstone (YSP?) I'd go with something warmer than 40, you can always unzip when it's hot, then you can use it on colder trips too.