I am thinking of buying either an ASUS Nvidia GTX 650 TI Boost Direct CU or Sapphire HD 7860 2GB DDR5 graphics card. I want to know, however, if my motherboard – an ASUS P5KPL-CM – can support these.
I concur; your motherboard has PCI Express x16 so you should have no problem.
you have PCI Express x16 Graphics Interface
you can get your graphic card to fit that PCI Express x16 slot but the speed will not be as with PCI Express 3.0 standard
Yes, your motherboard should support either of those cards – it has the required PCI-Express slot.
Just make sure your power supply is sufficient. Both cards should draw around 120W at full load. Although I don't know the rest of your hardware a 400-500W power supply should be sufficient to keep it in a good range of utilization.
Seems a old motherboard with last BIOS update in 2010. Checkout the power supply issue and the 6 / 8 pin to connect the graphic card.