The EM2420 requires an 8MHz system clock crystal for the Atmega and a 16MHz crystal to run the radio. It also uses a 32.768kHz clock crystal for sleep timing.
The EM250 and EM260 use a single 24MHz crystal to manage the radio and the network timing.
The 8MHz, 16Mhz and 24MHz crystals mentioned above must all be within +/- 40ppm to meet the 802.15.4 timing requirements.
To be compliant, they must meet these requirements in all operating environments and during the entire lifetime of the product — this means that you must take into account temperature drift as well as aging drift of the crystal you select.
If your product does not require 802.15.4 compliance, the EM250 and EM2420 are capable of operating properly with crystals of a much looser tolerance — down to +/-120ppm.