Hi. You don't want to cheap out on the PSU. That is a critical part of the system because if it goes, it can take the Motherboard and or other parts with it and you wil be replacing more than a PSU. This one should do: http://pcpartpicker.com/part/pc-power--cooling-power-supply-ppcmk3s500
As for the Memory question, no. Even if you did, it would run at 1333 Mhz as that is the slowest memory stick. The CL & Timings are probably different. Best to get a matched set od 2 x 4Gb sticks.