Cheers for the input thus far. Hmm tempted to go for a 30" Dell now too, that would certainly provide some serious screen real estate. I've been tempted by the dual Xeons.... but as you guys have pointed out, there's probably minimal mileage left in that socket.
I will probably go for a velociraptor based 900GB RAID 0 array, with a couple of terrabytes of system storage. Perhaps something like a 1TB Samsung F1 boot drive, then x3 300GB Velociraptors as a project array, with a further 2 GB for finished works (or could use a NAS setup to replace the latter for enhanced storage and redundancy).
The Quadro looks like a good choice because the GPU itself can be used to encode CS4 projects which would certainly help to reduce the time taken during what can be a pretty time consuming process.
Unfortunately MACs aren't an option.
What mobo/ram/cpu would you recommend?
How does this look: