Also, have you tried using CPM rather CPC?
Stuff that is mainly graphics is easy to just mentally filter out with the rest of the website chrome.
Just a guess, but the less "bannery" the ad is, probably the better chance it has of being noticed. Ironic, since banners got so obnoxious in the first place as an attention-grabbing mechanism.
What do you mean by the latter question?
Text-heavy banners get high click-throughs.