The phones will certenly handle the load. The 480i would just give you more screen area for custom menu's and separate phone lists etc... but if you dont need that then there is no real point paying the extra.
Having two separate networks is fine. The normal way of doing it is to have the switch configured to use VLANs. The phone can then be configured with a VLAN tag so its traffic goes over one VLAN while untagged traffic (the computer) goes over the default one. The main reason why this is the more common apporach is because it reduces the cabling needed in the office walls and you only need one cable from the wall to the desk since the pc cable goes to the back of the phone.