The Ultimate Guide for Game Completion:
Start the game off with your ordinary, crappy pistol.
-Stock up on points until you can buy the G36.
-Stock up on points until you can buy the Famas. (You may or may not want Marathon Runner. I got it for fun.)
-Save up enough points for OverKill.
-Save up enough points for the X M 58 Laser Cannon.
-The FIRST level that you have the OverKill/Laser Cannon combo you can generate 40k experience EASILY if you use it for the whole level. Hollow Points is another nice boost to have to this, but isn't necessary at all.
-At this point, it's best to get the GAU-8 or Pancor JackHammer. I recomend the auto-shotgun.
-From here on out it's easy sailing. You won't need any additional weaponry. Give your base 1k health and yourself 125. Get any powerups you can muster and have fun in teh zombie slaughter.
Great Game.