I run Warn winches on my trucks and actually found the need to upgrade from a 160 amp alt (came stock with my Hemis). It all depends on the load your winch will be pulling and how often you will use it. Based on my expereince and asking Warn and alt makers questions, a winch under load surges at about 400-500 amps. Thus, your alt and battery get a tremendous work-out the second you engage the winch. You may find your battery is short lived if you plan on using the winch regularly. My trucks were stalling under heavy loads (I also snow plow which requries about the same amps as a winch).
Check out DC Power Enginering's website. They have a lot of information about alts, loads, OEM issues, etc. Their alts can put out 260+ amps at idle, althhough the come at a high price. Since I use my trucks for heavy duty work, I took the DC plunge - their alternators are incredible.