
Originally Posted by
DaveP
Steam version should be the same as retail and will definitely work with retail versions in MP, but as a slight caveat some releases on Steam end up having bugs that aren't found on other platforms and issues therein, Retail is your safest bet in that regard. Also sometimes there can be issues with modifying steam games in terms of where the files go and the like, but usually it isn't a big problem; Best bet is to wait a week or two after the steam release if that's what you're after.
In terms of multiplayer, there are all manner of different forms of Deathmatch, Team DM, Capture the Flag, Capture and Hold, King of the Hill, Coop, PVE (Player Versus Enemy, AI), Warfare, Evolution, and a limitless amount of aftermarket modes scripted together by crafty community members. In terms of player limits, pretty much the only limit is how powerful the server is and its connection.