1. is that power supply much good?
OCZ is good but there are better and trusted psu out there
get this one http://pcpartpicker.com/part/xfx-power-supply-p1850bnlg9
2. what would be a 7850 that runs cold and also ships with a crossfire bridge?
i dont recommedn Xfire mid card, better performance : yes, but brings more problem than additional FPS. get single 7970 and you will be fine
no need to buy a new mobo too
3. the system is a gaming rig, would any more ram be worth its while? 8GB is good
make sure you dont fill your 645 SSD above 70%, it will reduce its performance significantly