You can use any crystal you like because the
PIC18F4520 doesn't have a USB controller onboard. Therefore you need an external USB controller and you follow the datasheet of that external controller to determine the required crystal.
YEs: max. clock speed of the PIC18F4520 is 40 MHz (32 MHz using the internal PLL), so 480 Mbit/s is impossible to achieve (you'd need 240 MHz at least for this) and 5 Gbit(s is way out of reach. It's all in the datasheet.