I'm looking into creating a setup on Rackspace Cloud that will allow me to autoscale my webservers (no db) on demand. Preferably using something like response time. I've read into configuration tools like Puppet/Chef, but I'm thinking I can just launch from prepared server images that are ready to go. Is there any tool out there already that can monitor my existing node response times and then launch or scale up new ones based upon certain variables like average X load over Y time? I see there are commercial offerings like Scalr, Rightscale, but how would I do this myself?