As time moves on each new video codec seems to be trading increased encoding/decoding complexity (and CPU time) for decreased bitrate required for a given video quality level. While systems like OTA will still be MPEG2 for a long time, services like Netflix, Hulu and Youtube will upgrade their service to quickly adopt new video encoding standards to get more for their bandwidth bills (and/or provide higher quality video).
This has happened before and will happen again with H.265/H.NGVC (for example).
If you purchase a "nettop" (i.e. netbook components in a very slim desktop form factor) those generally don't have a processor capable of decoding H.264 at higher resolutions, so they use a GPU that has been engineered to have "just enough" power to decode H.264, provided that you are using GPU accelerated decoding software instead of something like VLC. This solution will almost certainly not work with H.264's successor.
On the other hand, a fast general purpose processor will do just fine when the next version of the Flash/Silverlight that the internet video sites will use to decode their video.
This is why a HTPC that consumes streaming video is in a different boat than a HTPC that is just consuming OTA broadcast TV. The OTA HTPC has a fixed set of requirements. The problem is (IMHO) internet video streaming is the future of HTPCs.
I'm not advocating top-of-the-line processors for HTPCs, but I do think it is worthwhile getting ~$200 Sandy Bridge quad-core processors that are made using some of the latest manufacturing technology, idle at low-power levels, and when it's time to decode video they're fast enough to take on high resolution video just using software rendering. Plus, they contain SIMD functions (like Quick Sync) that future video codecs will almost certainly take advantage of.