You can OC the CPU, leave teh GPU at stock and run it perfectly fine off of 750w. You can even OC a little bit.If you are going for high OC or even OV then you will need 850-1000w.
550w for single 290x and OC. 750w for 290x crossfire @stock, 850-1000w for crossfire OC. If you OV you will need about 1500w, this is heavy OV such as using a custom BIOS.
I would go with the 278Q if you are planning on doing colour work. http://wecravegamestoo.com/forums/monitor-reviews-discussion/15027-best-27-monitors-2014-2560x1440-ahva-ips-pls.html
you will only need 850w if you are planning to overclock the 290s, if you leave them at stock then a 750w will do. You will only need 1000w-1300w if you are overvolting the 290s a lot.