404 is a substitute number, so that at level 100, Sesmic Toss/Night Shade will not be able to break the Pokemon's Substitute in one hit, allowing setup/less prediction. That 16n+1 theory is from the fact that residual damage (Stealth rock/sandstorm/etc.) will spare the Pokemon. You can certainly use this rule, but it is usually better to max HP on walls (not named Blissey), and more on atk/spe on frail offensive Pokemon.
@Skymin_Flower
So where can you get Pokemon XD then? And which console is the game on (and transfering the Pokemon...)?
XD Gale of Darkness is for gamecube, and although you can play it on wii i don't think you can transfer. you can transfer to 3rd gen games (eg ruby, sapphire) and plus you need a gameboy-gamecube connection cable thing to transfer. In short, it's a really long route i doubt you'll want to follow just for the dragonite. Great game though
Edit: as above :P