What is the meaning of speed grade marking on Xilinx FPGAs?
The speed grade has to do mostly with specified transistor switching speed. You can review the definition of -MIN, -4 and -5 speed grades in appnote 312, page 125 onwards.

This determines how quickly you can run internal clocked circuits. It will not influence the choice of your crystal oscillator, as you will use a PLL multiplier to generate the actual system clock from a realistic crystal frequency.