I've been busy monetizing my myspace site - Myspace Hun and I'm not exactly pleased about its CTR + eCPM, which aren't that good, but it's okay compared to some. Should I replace one of the squares with a leaderboard or does it not matter?
The only real way of knowing if it will make a difference is to experiment and see if it works. Change the layout for your site and use leaderboards for a week or two. Compare that with your previous performance and see which works best for your site. As far as I can tell the only main difference would be in the CTR. I doubt that leaderboard ads pay more than the rectancle ads. However if the 250x250 ad units don't have support for image ads (not sure if they do), you might notice an increase in CPM revenue.
They do, I use text_image on all of my advertisements. There's no reason to change my layout, the leaderboard will fit in just fine.