IAS Secondary Server

I am currently running IAS to control access to my wirless environment through RADIUS and Certificates.  The IAS server is an Enterprise Root CA which leases Certs to Computer Accounts.  This is based on membership of the laptop to specific groups, once a member of the group the autoenroll goes to the CA and leases a cert.  We are running 2 templates on the CA, RAS and IAS Server Authentication and Client Authentication - Computer.  For RADIUS clients we have a Remote Access Policy created to authenticate with "smart card or other certificate" through EAP.  Each Cisco WAP is setup as a RADIUS client.  IAS is Registered in Active Directory.
I have no issues with this configuration and have been running it for 3 years now...my problem is what do I do if this box goes down.  I know that if it goes down, my wireless network goes down with it.   I need to somehow put up a redundate or secondary box that can kick in if this server goes down...any help would be appreciated.  thanks.