Yeap, the 2400 pro won't do the job. Any 3xxx or 4xxx series will. Anything at or better than a 2600 Pro also will get the job done. I have a passive 2600 Pro and it works great. I did have trouble with non-accelerated HD content with even with an E2160 at stock speeds. Even overclocked to 2.4Ghz, the E2160 was just barely capable at 1080p, which leads me to believe the Intel Atom just wouldn't be able to pull it off period; even with GPU acceleration, because my E2160@2.4Ghz only had 30-40% shaved off the system load with my 2600 Pro.
Now my E8400 at stock 3.0Ghz just laughs at 1080p and asks for something else to keep it busy.