That largely depends on the format that you are decoding. Some formats will play fine in netbooks, especially those that are accelerated by the GPU. For netbooks, I suggest looking for one based on the E-450 APU. The E-450 provides better than Atom performance (and will handle most 1080p files when configured properly), while maintaining decent battery life.
However, I suggest going for a notebook if you're fine with the weight/size, as it is far more capable of handling a variety of current formats and bitrates (and possibly future formats as well). Most current notebooks should not have a problem with HD. The Llano-based notebooks are particularly interesting for me.
How much are you willing to spend? If you have the budget, CULV-based notebooks are about the same size as netbooks, and pack significantly more muscle.