Sorry for the noob question, but I'm really confused.. :( Is there going to be a third Ethereum? Will I have to manually upgrade somehow? Which new ETH should I use?
Thanks in advance!
At a very high level, you can think of the current version of Ethereum as V2.1 of Ethereum and Metropolis as being another name for version 3. Byzantium is kind of like version 3 alpha (a feature-incomplete version of version 3, except not really in testing phase) -- it is the first half of the rollout of Byzantium. The rest of version 3 is being rolled out in Constantinople.
As an end-user, because version 3 adds backwards-incompatible changes to the current version of Ethereum, you will need to upgrade your Ethereum wallet to one that supports Byzantium. If you don't, in the best case, your transactions won't be recognized by people who have upgraded (and vice versa). So upgrade, unless you expect a majority of users will not be upgrading. I expect pretty much everyone will upgrade.
You will not lose any ether or tokens associated with your existing accounts (unless it's due to user error, like deleting your private keys while upgrading, or programmer error; back up your keys!).