.325 chains are OK on 50cc and smaller saws. Typically the chains don't last as long, and will dull quicker on high speed saws than running a 3/8". From what I've seen, it's really not much of an "upgrade" to change them over on 18" or smaller bars, and on some saws they larger chains can really hurt power/performance.
If you have a saw with enough power to effectively operate a 20" bar, then the upgrade to 3/8" would be well worth the effort.
I've never ran a 346XP, but would imagine is would be at it's limits with an 18" 3/8 set-up. Our 55 is well suited for the 18" .325, but I wouldn't want to run any more bar on it, as we typically use it for smaller limbs where the fast chain speed is most desirable......Cliff