As you say...
Set the new drive as a Slave.
7200 and 5400 on the same cable, i have that setup no problems here...Although i do not copy an OS.
On the back of each drive there'll be jumpers little fidly things just move them to there correct postion. Look on the shiny side of the drive it will tell you how to set the jumpers up.
Thats it really, the BIOS should detect the setup and away you go.