need suggestions on banner advertising.

[dieselstation]

Hello, I run a fairly popular car wallpaper site, and i'm looking to diversify my advertising. So far i've only been using google and yahoo, but lately it's been all google since yahoo has started paying wayyy less.

I don't think it's a good idea to stay with just 1 advertiser though, so i'm wondering what i should do? What do you recommend for a website that gets 500K+ uniques and 2.7 million pages.

I would like to sign up for something that will let me choose by category, such as Automotive.

Is it hard to sign up for Tribal Fusion or DoubleClick? who else out there should i look into?