Reviews of Atom-based boards I have seen show that it doesn't do HD content well at all. 720p is hit and miss, and 1080p is just a no-go.
So it'd do DVDs and divx/xvid for you, but to play Blu-ray and mkv files you'll probably want to wait a bit and snag the new Nvidia ION platform, which pairs an onboard Geforce 9400 with an Atom CPU, and does full 1080p video with only 20% cpu utilization.
Also remember to divide Atom clocks by 3 when comparing to Core 2 (as a rough estimate, I choose Core 2 because most people are familiar with them). A single core 1.6GHz Atom might compare to a Core 2 at 500MHz using only one of its cores.