you have to complete the quest (The Waters of Life) to get access to the Citadel.
http://fallout.wikia.com/wiki/Portal:Fallout_3
No, When you complete both Fallout 3 or Fallout New Vegas you are not able to free roam the wasteland.
Yes
well...you meat Fawkes when you got to vault 87 to get the G.E.C.K. but you wont be able to have him as a follower until after the part where you are captured by the Enclave and then escape the base after talking to the president... Fawkes will be outside shooting enclave and from there you will head to the Citadel to complete the final mission together if u accept him as a follower when u escape
I can't give good directions on here, but if you just google "fallout maps" or "evergreen mills" or something you should be able to find it.
yes there is no reason you shouldn't be able to.
Now, you can't. When Broken Steel comes out you will be able to.
Nope. Fallout 3 and Fallout: New Vegas are completely different games, sure they are in the same title and the same game engine, but the configurations and in-outs are different for both games, so you cannot simply apply a mod you used for Fallout 3 and try to run it on Fallout: New Vegas. You may be able to find mods that match your Fallout 3 mods for New Vegas on an assortment of different websites all over the net.
just continue on with the main quest and you'll be able to do it.
no even if you install the goty edition of fallout the disc will still be needed to play them.
You can have as many saves as you like so you are able to create as many characters as you please.
In Fallout 3, there was a site called "Fallout 3 Nexus", from what I understand it was where everyone went for mods. Similarly, one was made for New Vegas named "Fallout New Vegas Nexus". You'll be able to find all the mods you could ever hope for there (just google it). Be sure to download the Mod Manager, it makes mods easier to use.
good graphics and plot, tons of weapons and ammo, and your able to kill anything in the game.