Depending on who you ask, If JBaz was here he would say OCZ though I believe Corsair or Seasonic would be the way to go. Considering what you have a 650W would be fine and still give you plenty of headroom.
Here is a Corsair TX650
there is also a modular version if you want but it's a little more expensive at least for now, Corsair TX650M
, either way its your choice.
I will also add that I don't think that your PSU is too weak more like it may be having some problems. Whether its a defect or otherwise is still up in the air.
If you have gotten the second MB in try first making sure you have the stand-offs in the ATX MB configuration or test it outside of the case on a piece of cardboard. after you have installed the CPU and heatsink, making sure the fan is plugged into the CPU_FAN1 header.
1)Install RAM in slots DDR3_A2 and DDR3_B2
2)Connect a HDD with the SATA data and power plugs
3)Put your GPU in PCIE2 slot and insert the 6-pin PCI power plug.
4)Hook-up the 8-pin power for the CPU, it should be in between the CPU socket and your PS/2 ports labeled ATX12V1.
5)Connect your 24-pin power cable labeled ATXP1 right next to your RAM slots
6)Connect the power switch for the case onto PWRBTN and GND pins on the area labeled PANEL1 next to your SATA connectors.
7)Plug in your monitor, mouse, keyboard.
Once you do all that plug in your PSU and turn the power switch on the PSU to the on position then press the power button on the case.
If your system boots your should be good to go if not let us know what happened and we can go from there.