I can't find a VNE for them but a post on Airliners.net lists the VMO, which is the fastest you're supposed to fly it in cruise, for the CRJ200 as:
0 - 8000ft 330kts
8000ft - FL255 335kts
FL255 - FL280 M0.80
FL280 - FL316 315kts
FL316 - FL410 M0.85
This AOPA article lists the VMO of the Citation X+X as Mach 0.92, or 547 KTAS. The corresponding IAS would depend on altitude and temperature. From the same article the updated version, called the Citation X+ the speeds are listed slightly higher:
VMO (max operating speed) at sea level to 8,000 ft | 270 KIAS
VMO (max operating speed) at 8,000 ft to 30,650 ft | 350 KIAS
MMO (max Mach number) | 0.935 MM*
* At 30,650 0.935M would be 549 KTAS