Best Firewall backup/replacement

hello,
We currently have a Symantec gateway 5420 firewall appliance in place on our network.
Everything is working great and we haven't had any major problems with it.  We feel that one day, that will change.
Everything on our network has been clustered or has some sort of a failover method should one of our servers fail, except for our firewall.  If it goes down,  we will have no internet access, and our websites will be inacessable as well as a number of other problems.
Does anybody have a recommendation as to what kind of software or hardware we can purchase to protect ourselves in the event of a failure?
One device that has been suggested is a "Watchguard Firebox® X 750e"  
another option would be to buy another Symantec box and have them running clustered,  but this would be a very expensive method.

any suggestions would be greatly appreciated.

Thanks