i'd also like to say Fallout 3 is one of the best games ever created. its an awesome RPG and i've got close to 80 or 90 total hours logged and im still playing. so much better than oblivion in terms of side missions, game play, story, graphics, immersion. i just cant say enough good things about this game, u have to experience it to understand.
and i saw army of two thrown around in this thread a little bit, its a fun game, probably can beat it in 6-8 hours if u have a buddy to play with, but it is good and the sequal was recently announced and it looks crazy good so u might want to get AoT, its about 25 dollars i think.
finally, to answer ur ? about the MGS4 story, i did not play any of the previous games, so yeah i was a little lost in the story but it was still compelling and if you go on youtube and search for videos theres a ton that brief u on the MGS story up til end of 3. some things u may not know about MGS4-half hour cut scenes, largest game ever created (took up all the space on a blu-ray disc and they even had to cut some things out), awesome sound track and graphics, tons of ways to play a mission-you can even beat the entire game without alerting any enemies or firing a single bullet.
sorry this was so long lol