450W PSU - This won't run anything. You'll need at least a 600W PSU for the 280x. Buy a 800W if you want to use 2 of them. Expect to use 250W - 300W per Graphics card and 150W for the system.
Yes , you can use a 1x port for a Graphics card. Get a 1x-16x riser. Read here for how to build a rig --> http://www.coinminingrigs.com/