the hd5x8 series is open while the new hd4x1 series is closed.
i have not heard much on the new models but what i have is that for the price they actually sound pretty decent.
the hd5x8 series will have a bit more soundstaging and perhaps be a bit more neutral overall. the new hd4x1 series sounds like it has a a little less soundstaging (though still decent for closed cans) a bit of isolation from being closed (but not heavy isolation) and a little more mid bass/sub bass push without being heavy. build quality seems to be a bit more plasticy and cheap feeling but that is expected at $110 and below.
since i know more about it and soundstaging is generally important for most gamers i would say the hd558 (thats not a typo of the 518) would be the one to go for. it has the nicer pads (over the cheaper 518) and some better build quality with a similari sound (to 518 and 598).
if isolation is important to you (loud noises near you) then the new headphones might work for you as well but i cant say too much about them. they are too new so there is not much to be said about them yet.