I bought a hexacore MacPro about a year and a bit ago, mostly to process 5DII RAW files (heftier than what you'll be producing) but also to do a little bit of video and a fair amount of multitrack audio recording, effects, that sort of thing. SSD boot/applications drive, RAID 0 WD Caviar Blacks for main data, internal AND external Time Machine backup onto single drives because I trust RAID 0 about as far as I can throw a cannonball for data security. Expensive option, but I wanted the internal upgradeability of drives and RAM (houses 24 gigs), and it makes using even slow software like DxO optics a real joy, and even playing around with huge TIFFs in photoshop gets fun because you don't have to wait. Recodes/renders HD video in a snap, too.
Is it overkill? Certainly. I use my Macbook Air 13" on the road for light editing tasks, and it works alright; a year newer but significantly slower. A good monitor is almost more important than all the hardware - not much point to speed if you can't accurately see what you're running.
Before that I was running a 4 year old white macbook, which was getting really annoying really fast.
Right now, I'm not sure 100% sure I'd get the MacPro per se - a 27" iMac (maxed RAM and SSD main drive) with a Thunderbolt high speed disk array is an attractive alternative proposition - mostly I need/want large fast data drives for the audio recording (24bit/192kHz multi tracking gets really big pretty fast, more data throughput than HD video)