Be careful purchasing a USB tuner for such an underpowered system.
Most USB TV Tuners don't have an internal MPEG-2 encoder but use software encoding instead. This uses your CPU which takes a LOT of power, something the single core Atom doesn't offer.
The only USB tuner I know of with a dedicated MPEG-2 encoder is the Hauppauge WinTV-HVR-1950
. It has fairly decent reviews by the looks of things.
I know most mini Atom systems don't have PCIe slots, but if yours does, I would strongly suggest you get an internal card instead.