FCP handles many video formats and since version 4.5 HD codecs have been part of the program.
You can edit HD without a capture card but it is advised that you use a card for capture and external monitoring. You will need a monitor capable of displaying the flavour of HD you wish to edit. Also external hard drives capable of HD data rates will be required.
Start by looking at the options in the Easy Setup and then refer to your manual for details explanations of the various editing codecs.
This ALL depends on the format of HD you will be editing. HDV, AVCHD, DVCPRO HD, XDCAM...all can be editing using firewire drives, all are capturable over firewire. And you can use an iMac. You don't NEED a capture card, nor do you NEED an external HD monitor. This all depends on what you will be editing.
FCP is a very versatile editor. You can edit DV up to 2K Red files. The things you need to worry about are media drives and the computer you are going to be using...and all this stems from that FORMAT of HD you will be editing. There are so many.