What kind of camera will you be using, and what format do you intend to work on, AVCHD, XDCAM ,ProRes422, 10bit uncompressed, etc and whether your 720 or 1080. Being you said amateur i would assume ACVHD, which would just use a little more CPU being its a pretty good compression. Not familiar with iMovie and what it works with but some of the higher formats are hogs (10bit uncompressed 1080 6min video ~60 gb

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You asked about 4 and 8 cores earlier, i wouldn't think iMovie would benefit from multicore processing like FCS2 (compressor) does. Use that money for some good HDD's, with HD you'll need it.
And pick yourself up a BD-RE disc to do some practice burns to make sure it works in your BD players and looks the way you want.