good question, i liked Fifa09 as much as fifa10 all be it the game play is better over all the way it plays is the same, the thing you'll have to remember is, your teams in the lower leauge are dogs, learn how to do the chip shot and run straight at the keeper, it was the only way i could get a goal in the English2, and it was very frustrating!!
Halo3 is an awsome game, i played it to death, so much so that now i dont want to put it in through fear of neglecting my family haha!!
Call of Duty Modern Warfair2 is.... FANTASTIC, it is however a game pitched at online gaming, the main game will take you about 6 hours to finish, however you will have hours of fun online, word of advice for that dude!! dont use the head set if your not online with your mates in a "Party" because the tubes on line will do your head in!! oh and find the setting to stop the voices coming through your tv!!
I also play Forza3 its a car racing game and the graphics are amazing!! if your not that good at car games it will help you loads with assisted braking/steerging/traction and Abs, but you can if you want switch them all off!!
from the above game you have done what i did with MW2, i wasnt a fan of the older COD so i bought COD4 for about a tenner and fell in love with it, so if you want to give it a test run buy the older ones.. COD4 is still a good wee game and online is good too.
Fifa09 good shout, and if your in to car games get Forza2 first see how you like it!!
Good luck and welcome to the world of xbox. you wont look back lol!!!