Installed new video card, system reboots

your current graphics card settings is not set to the correct level to match your system, you can download ATI Catalyst software from this link:
http://support.amd.com/us/gpudownload/Pages/index.aspx

after installing, you may have to restart your computer. when you restart, right click on the desktop and select (ATI catalyst), there you can adjust your graphic card features or you can use the recommended settings the software have adjusted for your graphic card by clicking on restore settings/defaults.

you should not encounter problems once you have installed the ATI software.