From the ABC article:
Still, no one knows how much beer is needed to reap the benefits [of xanthohumol]
And a quote from another article entitled "Scientists Reccomend 120 Gallons of Beer per Day":
[Xanthohumol] is so rare that a person would have to drink about 120 gallons of beer -- or roughly 1,300 12-ounce bottles -- every day to reap the benefits.
I'm just being overly picky. Anyway, both articles are interesting. Basically, don't count on beer for your antioxidants, although we're learning more and more about the health benifits of beer.
Oh, and if you read the first paragraph of both articles its funny that they both refer to Homer Simpson :).
(PS> The first many Google results for Xanthohumol are informative FYI)