I highly recommend you max punishing blade and ferocity very soon. Early zerker DPS comes from the extra swings and double attacks consistantly. I maxed flurry soon myself, but held off on raging flurry for awhile until I got the rest of vets fury, punishing blade, level of each defense AA and physical enhancement so I didn't feel so gimp.....there's a good forum thread on goberserker that parses the AAs as you get them. The extra swing/double attack AAs netted 2.5-3.8% DPS increase per lvl.