Grey screen following install if Tiger


Hi All
I wonder if anyone can throw any light on a problem I have. I have just installed Tiger on to an external firewire drive.( I did this to check out Tiger before I decided to upgrade my main hard drive.)The install was completed successfully but My machine will not start up and only shows a completely blank grey screen. I have made sure that the external dive is Ok with Disk utilities and is the only piece of kit connected to the firewire ports.
Any help would be greatly appreciated.
As stated My machine is a Power Mac G5 Dual 1.8GH 1.5GB RAM Machine fittred with a ATY RV350 Display card ( THATS WHAT SYSTEM PROFILER SAYS) The firmware and video drivers are the latest available.