Switching to iMac have storage questions

I am a longtime PC guy who just ordered a 27" iMac with 2TB internal HD. My passion in life is photography and I have a lot of photos. On my PC I have two internal 1TB drives which are mirrored and one external 1.5TB drive. My photos files keep growing and that is why I ordered the 2TB internal. I would like to have a 2TB external to have a second backup of my photos. If I get a 2TB external drive will the Time Machine function make a mirror copy of everything on the internal? Should I use my current 1.5TB external as a backup of all my photo files and use a second 2TB for the Time Machine?

Since I have just ordered the iMac and won't receive it until after Christmas I want to be prepared. I really don't know much about the Time Machine and would appreciate any advice. Currently I am thinking I won't transfer all of my photos on my pc internal drive over to my iMac internal drive, but I don't want to ever have only one copy of my photo files as I know some day I could/ will have a hard drive failure.

Thanks in Advance

Yes, you can use Time Machine very easily to backup whatever you want to an external.

This is from Apple's website (applies to OS X v10.6):

Summary
With Time Machine, Mac OS X v10.5 Leopard makes it easier than ever to back up your files. You don't need to burn DVDs or CDs or manually back up individual personal or application files such as you might have done with previous operating systems. You can restore Time Machine backups anytime you want, even while started up from your Mac OS X v10.5 Leopard installation DVD.

Products Affected
Mac OS X 10.5, Time Machine
Time Machine backs up to either an external disk (such as a USB 2.0 or FireWire-based third-party drive), a secondary internal disk or partition (if your computer supports additional internal drives), a Time Capsule, or an available Mac OS X Server version 10.5 volume. Although you can still burn personal discs in Leopard whenever you want, Time Machine itself does not back up to burnable DVDs or CDs. One purpose of Time Machine is to protect your data in the case of issues with your Leopard volume, so it only backs up to a separate volume.

Be sure to poke around on there and here for more info. And don't forget that your external will need to be formatted to Mac OS Extended (Journaled) to use Time Machine.

Hi Jr, and welcome to the forums (haha, I've always wanted to say that, even though I just joined today, )

I had similar questions, particularly in conjunction with Lightroom (not sure what you use to manage your photos).

Anyway, if it's helpful, here was my confused attempt to understand Time machine v. archiving, in addition to Lightroom's catalog back-ups (like you, my only real concern is photos, though I'm nowhere near capacity as you, having only begun shooting dSLR recently); YMMV: Lightroom Forums: Lightroom, Time Machine, and archival strategy...share advice for beginners