you can connect any number of internal & external hard drives as long as your bios support & available connectors..
but you cant run more than one OS normally, for that you have to go any of virtualization products like
virtual pc or virtual box, or vmware..
virtual pc & vitrual box is free & limited features..
if you need more option go for VMWARE , which give vmware server for free(needs free registration)
or vmware workstation which is commercial product..