If the motherboard is from 2006, it most likely has a pcie slot, although there were still AGP mobo's being made then, but it could very well lack any dedicated AGP or 16x slots; depends on the model and the features it has. I would find out what model it is, you can use a program called CPU-Z to find out the model number.
Is he right? Will or won't the graphics card work in that motherboard. I'd like to know the answer to that question.
Need model info.
For performance, it also depends on the application being used, if its more CPU or GPU intensive. An old processor can hinder the performance ability of a more modern, fast graphics card in instances of where the program is bound to both cpu and gpu cycles. Some applications, like General Purpose GPU programs, can run independent from the CPU and thus not bound or hindered by a slow CPU. I can put my high end GTX 570's that I have in my i7 PC and dump them into my old e2200 celeron system and have the same performance in GPGPU apps, but gaming would suffer tremendously.