A CRJ-701 Captain tried to reset the IRS in cruise flight when it annunciated some errors; resulting in the loss of attitude and heading information. An emergency was declared; and they were vectored to a safe landing.

Narrative

We were at FL360 and ATC had just slowed us to 250 KTS. The left side FMS started to display IRS reverted MSG. After I scaled down on my MFD the left and right IRS cross displays were more than five miles ahead and to our left from our GPS position. We were in straight and level flight and now at a speed of 250 KTS. I thought I remembered reading something in our initial Tech Briefing that the IRS could be reset in those conditions. I conferred with my First Officer. He agreed that under that condition it can be reset. We verified our position with the arrival and were prepared with VOR backup; just in case. I reset the IRS and we lost all attitudes and heading information and the autopilot kicked off. I assumed manual control of the aircraft. We requested descent out of RVSM airspace. As I descended and turned; I knew I would need protection for altitude and heading deviation. I told my First Officer to declare an emergency. We were vectored into [destination] with no further incident.