Any that suits your playstyle.
Not on the regular game, if you really want to, get the game for PC. I'm sure they have a mod for it on the Skyrim Nexus or on Planet Elder Scrolls.
If you mean playing it then yes, if not, then it's up to you.
Play It?
I played both, and I would recommend Skyrim. There is simply much more you can do in it.
No.
Skyrim. There's tons more gameplay and has the ability to generate an unlimited amount of quests.
as long as you have 2gb of ram, which is how much Bethesda have restricted Skyrim to use on any computer ( without the use of a mod), you should be able to run Skyrim, mediocrely
You can start the Jagged Crown quest, and take the crown to the leader of the opposing faction. So instead of returning the crown to Ulfric, take it to General Tullius instead. However once you complete that quest, that's the point of no return.
If it has factions installed, you type this in: /f create name Replace "name" with what you want your faction to be called.
It should tell you if you need to update
You can "win" Skyrim by finishing the main quest, faction quests, or all the quests. However, you can never spiritually win Skyrim because of the sheer amount of content and ways to play the game. There're a ton of areas to explore, spells to unlock, weapons to craft and enchant, and potions to make.
There's nothing to join, you just buy the game and start playing.