Well, you could use a dual molex (four pin) to six pin PCI-E cable.
The GT 240 would work alright with an Athlon II X3 or Phenom II X3, probably medium settings on newer games and high settings on older titles.
What is your brand specifically? Because a 580 W PSU without a single PCI-E six pin adapter is kinda dodgy, unless it's old enough.