If you want to test non-mobile offers then try setting up on Right Media Exchange instead. It's cheaper and you can use flash banners and set CPA targets and even do CPC buys if needed -- no demo targeting though but with a $0.15 CPM bid you should see some volume.
If you are going to use the CPC self-serve then don't waste your time testing every niche, open up all the popular pages on MySpace and grab the iframe code for all the banner sizes -- the placementID indicates where on the site the banner loads and you'll want to throw in all the popular 728 and 300 sections (Can't view this profile page, browse page, news, music, videos, logged-in homepage -- skip the profile pages it's mostly AdSense defaults when viewed in the iFrame). MySpace is a huge CPA network for advertisers and you'll see based on both the order and frequency how each offer is doing -- you'll see all the usual suspects there and you'll see some banners 20+ refreshes in that you know not even to bother with unless you are banking on the demo target. If you want to really test out the demo targeting create an excel sheet with a list of all the offers for only the homepage placementID (when you are logged in). Again, open up the iFrame in a tab and create 1-2 dozen MySpace accounts with different demo targets (female from kentucky age 16, male age 22 NYC, etc) and clear cookies and cache and go through 20+ refreshes in each and log all of this in Excel. You'll get some good trending data out of this and it works well to find the offers you should be promoting and the type of banners you'll need for each demographic. Don't try to do a regression on the data since all the performance ads default without demographic targeting, your goal is to see where the order of offers changes and when you see a new offer that does not appear outside of other demographics.