I have an older 3.33GHz quad (not hex) with 16GB of RAM, ATI 5870, and an Areca RAID card for an 8-bay tower of drives, and I do pretty well with HD footage. I use mostly After Effects and Premiere CS5, and I also have a GTX285 which I almost never use despite the CUDA feature. (It doesn't help me in After Effects at all, and I do fine with Premiere and 5870.)
RAM is cheap enough now that I'm about to up my RAM to 32GB, so I'd get as much as you can afford from a place like OWC. My RAID3 sees sustained data throughput speeds of ~700MB/second read, ~750MB/second write. I also have a 3x1TB RAID0 internally for my scratch disk, and it gets ~330MB/sec read/write sustained. The OS is on the stock 640GB drive. This setup works incredibly well for HD.
If I were building a Mac today for HD editing, I'd get the 3.33GHz 6-core, 32GB of RAM, and the same RAID setup, maybe using SSDs instead of 7200rpm drives in my internal RAID. I'd definitely try to pick up a refurbished Mac Pro before buying brand new. You could get that
refurbished 2010 quad for $2119 that's been sitting there for weeks, and upgrade it to a 3.33 or 3.46 6-core and save a ton of money right there. (I would have snagged it long ago if I didn't love my current system.)
I built this up over the course of 18 months, but started with just the 4870, 16GB of RAM and the internal 3TB RAID0. In that first setup, I was able to effortlessly edit DVCProHD 1080 footage into a 104 minute feature length movie without even rendering most of the time, even with effects on the layers. Eventually I got the 5870 and GTX285, and then built the external RAID when enough extra money came in from edit jobs. The point is, you can build up slowly and make it work, if you make good use of your internal drives and external backup drives.
