first, does it make sense to upgrade from 4.2.1 to 4.3? everything is just working so nicely... what are the advantages of 4.3 on an atv2?
next, i have a windows home server with all my data stored on it, i know that smb shares are not yet supported in the beta 4, but i also have an dlna server the ps3mediaserver running on the whs, so what do i have to do that this works under media>Manage Shares on the atv2?
There is one HUGE disadvantage in upgrading the iOS to 4.3. That is at the moment the only jailbeak for iOS4.3 is tethered so you have to use your PC/Mac with ATV2 to boot everytime you reboot or power off/on the ATV2. Hopefully this particular issue will soon be rectified when an untethered jailbreak becomes available.
Another point is that XBMC is reputed to have issues running on iOS4.3. Whether this matters depends on whether you are using that as your media player, or the FireCore media player.
My recommendation would be to stick with iOS4.2.1 until these issues are resolved.