> I often wonder why they design/ship x1000,x5000 and A1222 dual core
> as OS4 dream machines, instead of going high frequency single core ...

X1000:
There has never been a single-core PA6T on the market.

X5000:
Price difference between dual-core P5020 and single-core P5010 is about 42 USD, which is less than 3% of total board price. And both are 2.0 GHz max.

A1222:
Price difference between dual-core P1022 and single-core P1013 is about 6 USD, which is less than 2% of total board price. And both are 1.2 GHz max.