Q1) Yes correct x500 only
Q2) As you probably know, there are other things in GPUz that distinguish the card from an x500 and an x300 - like speed, memory, ram type, bus length etc. So if you post in the wrong area, and people are competing in that area, they'll notice you've put it in the wrong category and it'll be flagged and moved.
Q3) Vigilance of people competing in that area.