I was thinking about doing a cowboy character.

Is there a decent set of armor/headgear that has a "western" look to it? Powered armor and combat armor look too futuristic for what I'm thinking of.

Only thing I can think of is a cowboy hat. After all, what IS a cowboy without his hat (or a cowgirl without hers, in case of females).

Well yeah, there's all sorts of cowboy looking gear available. It won't necessarily provide you with best protection, but you can easily look the part. Reinforced Leather Armour is probably your best bet, with some sort of cowboy hat. And there's a few of those to choose from. There's also a couple of good perks you can get when wearing light armour like leather that will really help you out a lot.

There is a duster in Primm similar to Simms in Fallout 3. I would get that and a cowboy hat.

Decent?

Desert ranger combat armor + ranger grey hat

In my opinion, the Ranger combat armors (both the NCR and the Desert) do not seem like they feel cowboy, whereas the dusters do personally.

Yeah the dusters are definitely cowboyish, but they don't offer much in the way of protection.

Ouch, none at all in fact. They have DT of 0.