i have thought of preordering this on PC but since it's been announced, when does the PC specs for a new game usually be be revealed?

A few months before the release

Its a updated engine, CE3.4. It still runs on current consoles. but the more open enviroment(apparently) suggest it has been optimized, id say the pc-specs will be roughly the same. maybe a bit higher due to the hardware developments(7970/680 etc)

If you can run the Dx11 patch with the high res texture pack you should be fine i think.

In terms of performance this'll probably be a Crysis Warhead scenario. Improved visuals with less performance drop due to increased optimization.

Warhead did that in one year. C3 has two years to do that. No sweat.