When you trying to arrange 500 downloaded photos in the order that you want them it is not the same time drag then it is to cut and paste.
Maybe you should look at
why you are trying to manually arrange 500 photos in a specific order, which sounds like exactly the type of tedious task computers are designed to avoid....
Various options:
if you want the best photos to show up first, you can rate them. It's easier to hit apple-3 to add three stars than to hit apple-x and apple-v to cut and paste. Then you can tell iPhoto to sort by rating, top to bottom. And the rating can be used for other purposes as well.
Or, iPhoto will automatically sort them by time and date--if the time and date are wrong, then better to use iPhoto tools to fix that once and for all, than to manually rearrange.
So--what are these 500 photos and exactly why are you trying to re-arrange them? What order do you want to browse them in? Cut-and-paste is just a tool to get to your ultimate goal--maybe if you describe that ultimate goal, someone here can provide a better path than manually re-ordering.