Is an old dual-core processor enough for a HTPC?

It is about an old Core 2 Duo E6700 (@ 2.66 GHz) The soon included board is replaced by an Intel Core 2 Quad, since the board supports only dual cores.

Now I wanted to continue using the board with the CPU as HTPC. Would that be a good idea or not. Because when I watch YouTube videos in Full HD, my CPU is almost 100% busy.

4 GB of RAM are used but only 3 GB can be used (the board does not support more)

Would Netflix run smoothly?

It was a nVidea 9600 GT installed.

In general, it is not the CPU that matters, but whether the hardware acceleration chips are available. For a Core2Duo very questioned. But if you replace the board, make sure it supports HD / 4K video playback. The CPU is secondary here.

My new board only uses 40% of the Quadcare CPU @ 2.33 GHz.

Does not mean anything. FullHD is 1/4 of 4K. 4K would therefore burden the CPU to 160%. But just. Special chips that use the H264 codecs can then suddenly deliver 4K video streaming with very little CPU load (10-20%).