NWA first class is very lacking on most demestic flights. Some DC-10's and 747's on domestic routes have nice first class cabins, but most are small with only non leather seats and a small seat pitch. NW partner Continental by far has a more superior product, and if I was you and you can, go with CO instead.
Upgrades are usually pretty easy on NW, depends on the city of course. As a silver elite, I am about 85% succesfull. I am not sure how much it would cost to upgrade but I would think at least $300-400 dollars each way.