Crossovers/Events/Major Stories:
#1-6 - The Five Nightmares
#8-19 - World's Most Wanted (Dark Reign)
#20-24 - Stark: Disassembled
#25 - Heroic Age
#33 -> #500 - Legacy numbering shift
#503-509 - Fear Itself
#510-515 - Demon
#516-520 - Long Way Down
#521-527 - The Future
Legacy numbering explained:
#1-332 - Iron Man (1st series) #1-332
#333-345 - Iron Man (2nd series) #1-13
#346-434 - Iron Man (3rd series) #1-89
#435-466 - Iron Man (4th series) #1-32*
#467-499 - Invincible Iron Man (1st series) #1-33
*It appears that to get the desired "Legacy" number, editorial did not count the last 3 issues of Iron Man (4th series), which had been rebranded on the cover as War Machine: Weapon of SHIELD. Interestingly these three issues were used when Legacy numbering was adopted during Invincible Iron Man (3rd series).