Honestly, if everyone has 800x600 it would bring everyone back to .. an even skill level.
800x600 does matter quite a bit when it comes to duels/2v2s or something. Its just the split second where you see your opponent and they do not see you. Same goes for bullets, easier to dodge. (Any bigger than 2v2 gets more messy and so its harder to stay alive there anyway. No big difference there.)
I just vote for everyone to have it so it makes the game fair. Not only the ones who can *get* 800x600 into their hands >: O

About levels looking more ugly, no clue.