Ned I would try to get about 1" of clearance...any closer than that you are probably into carpet-melting territory (just my opinion based on past experience...YMMV)
For added safety margin you could install a piece of radiant heat shield (the aluminum dimpled type, mounted on small standoffs) or, a material such as this:
http://www.heatshieldproducts.com/in...eat_shield.php