System?

The "ideal" system is something you will be chasing, it's basically a budget/performance balance. The more money you can throw at a system the more power it will have but it's very much a case of diminishing returns. You could go upto 2 3.6ghz for an extra £390, from 8Gb to 12Gb for an extra £2740. You could add a nice high bandwidth SCSI array for a few grand more. So long as you accept that between you ordering one and it turning up something faster and cheaper will become available.

I am using a system pretty similar to what you describe. Replace the SATA 7200 with Ultra320 10k drives, and that's about it. Does it work OK? Sure! Is it "that" much better than your off-the-shelf workstation? Not so sure. Oh, and also - I had to pay $200 for a fancy Lian-Li box, because all that stuff wouldn't fit in a standard ATX case.
What would I do if I could add or change ONE thing? I'd like something QUIETER! All that stuff requires lots of cooling, and lots of cooling makes lots of noise. If you work all day on it that noise becomes a real nuisance, and I believe degrades my concentration and indirectly the quality of my work.
G Faris