The 500x80 pixels existed since the site first started, changing it might screw up some people. The buyable banner are newly added and they are standard size I know.
I might change the 500x80 in the future (adding a check for sizes to determine resizing it or not as suggested by Skotlex). But not until I switch server (soon!), need more cpu to handle the load. It's not an excuse really, the cpu load right now can go up to 3-7 at peak time. I don't want to stress the box.
