probably. I have yet to see any direct comparisons, but reviews have showed it to be surprisingly powerful. Light gaming on wow and CoD 4 at respectable frame rates as well as the requisite 1080p playback.
I will actually be building a rig like this shortly. the whole rig consumes 41 watts under load, so it seems like a much more efficient media rig than my main gaming build.
I think the E350 would definitely be worth it. In term of raw CPU power, the Sempron might be equal or slightly better (better for sure if you unlock the 2nd core), but unlocking the 2nd core will undoubtedly increase the power consumption (probably to the 65W of it's dual core Athlon II siblings), moreover this wattage will be for the CPU only, the GPU on the MB will require extra where the E350's 40W include both CPU and GPU.
For the GPU itself, the ATI 6130 in the E350 is considered ~33% more powerful than the 4250 on the Biostar board, this could make HD playback a lot smoother and it supports DX11.
With the Gigabyte board, you get USB 3.0, but with the less expensive Asrock board, you get eSATA, I don't know if you prefer one to the other.
Finally, the SATA on the E350 boards are 6Gbps, not 3Gbps; this could be useful if you decide to get an SSD (which, ironically, might cost as much as the HTPC itself).