Four and a half years ago, I started HD with a G5 dual 2.5 and a dual fibre channel ATTO 4TB RAID so I was doing uncompressed 10 bit 1920 x 1080.
Today I am still using the G5 but with HDV/ ProRes workflows common, I can use FW800 drives. XDCam experience is similar. It all comes down to render time. I am looking at the upgrade path and at this stage, I can’t see any reason not to wait a few months until the price drops on the 2.9 Octo Nehalem. I will basically buy as much grunt as possible to get the longest life out of the system, like I did with the G5.
My RAID is still useful although I might swap out the 14 x 250 gig drives for a bunch of 1 or 1.5 TB drives.
Only other choice is switching to AJA from Blackmagic and also getting the most graphics card grunt. It might seem expensive to buy the best, but how much does it cost to have to change systems more often?