Depends on the game engine - some game engines will accept textures that aren't powers of 2.
I thought that Halo CE was one of them - isn't the Assault Rifle texture something odd like 640 x 640 pixels?
Printable View
Nope, it's 512 x 512.
Not all have to be power of 2 either :P
Interface bitmaps don't have to be powers of two, but... that's for HUD stuff, and it's not that hard to convert to powers of two.