Left side - right side - top or bottom? Small banners like you have now at top right are just fine.
I can see an area at bottom right or left for this type of exposure too.
Sell them ourselves or use a network? Which network? Aren't you large enough to create your own ad network? I would think so.
I really think you should give some serious thought to creating the WebmasterWorld Ad Network. You've got a large group of professionals who would be eager to advertise. Not only that, but you have the talent and resources to do so. Of course there will need to be plenty of human intervention but I believe you could set a new benchmark for quality in an ad network.