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I just went through pretty much the same thing when I got my Mac Pro a few weeks ago. Booted off the stock 320GB drive and pre-installed system to test the machine, then installed some software updates. Installed a Western Digital 640GB drive and used SuperDuper! to clone over the stock system to my new drive.

If I had to do it over again, I'd do a clean install from the bundled DVDs instead of the clone. I had a few wonky problems with my system that were largely fixed by re-running the 10.5.3 Combo updater on my cloned system. If I run into any other weirdness I'll probably turn to an Archive Install next.

Otherwise I love my Mac Pro (single quad 2.8GHz). BTW, it ships with two install DVDs: disc one has OS, bundled software (including iLife 08), and some optional installs. Disc two had the developer tools.
 
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