I'll bet that since your PSU is a Dell OEM that it:
A: Doesn't have the necessary connections for things like the video card which takes 2 PCI-E cables
B: Is a cheap POS. Personally, I wouldn't put 940$ worth of hardware anywhere near that PSU, all it takes is one bad surge and your whole system is fried.
Makes more sense to get a reliable PSU from a brand like Corsair, XFX, Seasonic or Antec to make sure that your system gets reliable, clean power.
Also, since you plan to overclock, that Dell PSU won't allow it. Makes more sense to get the PSU now and get everything hooked up than to realize that it won't work for you later on and have to unhook it from everything in the system.
Also, why would you get a 560ti? All of the current gen cards have been released and you can get more performance and efficiency buying current gen.