Q: Moving from PC to Mac, alreday have files backed up on Seagate external drive.  Can i 'just' load files from external drive or does the drive need to be set up for a MAC? thanks

You can copy your files from the external drive to the appropriate folders in your Mac's Home directory - Documents, Downloads, Movies, Music (put your iTunes folder here,) Pictures.

The EHD can be a Windows format, OS X can read from it however it will not write to it natively. Once you have completed the migration you may want to reformat it and use it as a Time Machine backup drive.

If you haven't already please look over Pondini's guide on using Setup Assistant, if the PC and new iMac are on the same network migrating is extremely easy. Please read Lion or Mountain Lion Setup Assistant tips

Check out KB Article:  http://support.apple.com/kb/HT1408 How to transfer data from a PC to a Macintosh

Have you tried Migration Assistant? I believe that facilitating the transition from PC to Mac is its purpose, it may be able to help with moving your files.