I think you have a typo as shouldn't it be micro ATX rather than ATV? Unless you want to be riding in the backcountry.
And if you want small then get MiniITX as they are smaller. If you get one that supports socket 1155 aka Sandy Bridge then you have a CPU with an integrated GPU that is more than fast enough to handle any HTPC tasks. The CPUs start at about $130 for an i3-2100.
Zotac specializes in small mobos and Asus and the other traditional mobo makers also make mobos in the mini ITX form factor.
Why do you need any PCIe slots? I am pretty sure that all of these types of boards will only come with PCIe slots.