I will be touring NYC - Seattle this summer using the Northen Tier + Lake Erie Connector route.
I'm looking for a good sleeping bag to keep me warm on those cold and not to cold nights. Im not looking to spend more than $150 for this. What temperature rating should i look for since I will be encountering some varied temps thru that route? Would a 40 deg rated bag do or should I go lower to 20 degree?
We're currently looking into bags for a trip that whose more extreme cold will be about the same as yours. You might be able to get away with a 40 degree bag, if you're prepared to add a liner or some other compensation for when you cross the Rockies. We're planning to go with something rated in the -1C to 35F range (some are rated metric, and others in Fahrenheit; but I'm looking at freezing plus or minus a few degrees). There are serious advantages to avoiding the 20 degree bags, including cost, weight, bulk, and surviving the hot nights. If you go with the 40 degree version, you can either mail yourself a liner or buy one when you are closer to where you'll need it. But I really wouldn't want a 20 degree bag for the Lake Erie connector or the midwest and prairie portions of the Northern Tier.
An additional source is backcountry.com. They have a large selection, and reasonably easy navigation. The four sites I've searched are mec.ca, rei.com, ems.com and backcountry.com. The latter seems the most comprehensive. Once you narrow down your choice to a single bag, you can see whether it's locally available, or use google shopping.
Unless you are sure you can keep the bag dry at all times, I recommend synthetic, not down. Weight should not be significantly higher than 1 kg. Packed volume not significantly higher than 10l.