I'd be finding out what exactly Rackspace will warranty with the service/software.
Anti-virus software does not prevent someone from breaking into your machine. It prevents someone from "infecting" your machine with a bit of software. Secure passwords, good firewall rules, secure web code, and keeping up on security patches and bug fixes will do you FAR more good than an 'anti-virus' software on a *nix machine.
Tips I've found useful:
Servers: if you must allow access to the machine for configuration, use SSH. Change the port from 22 to something else. (Takes care of the majority of script kiddies). DO NOT ALLOW ROOT LOGIN FROM SSH. Make users use sudo. Request that the db server only have a private IP address. Make sure you can access that db server from your web server. Secure the db server in the same way. WRITE SECURE CODE. Keep an eye on your logs.
Firewall: There are plenty of firewall guides out there. Rule of thumb: deny everything, then open up only what's needed.
Always assume that the hosting company will not be responsible for ANYTHING, unless it's written down in a signed contract agreement. Make backups. Install (and properly configure) things like Tripwire, Snort, Nessus.. These will help with notifying you about a possible break-in on your machine. Make backups. Make backups. Make BACKUPS.