I rebuilt a Core Duo machine with a view to smooth preview in Vegas 9,- i7 920, 16Gb RAM, RAID 0, 2x 8600 cards.. nothing doing: you still get some stuttering particularly where transitions etc. are used.
The extra power has, however, made a big difference to rendering times.
The best hope seems to be for Sony to make use of the GPU or for GPU Decoder to become usable. GearShift is a good idea though I’ve found it produced an irritating banding or moiré effect in preview.. unless there’s something I’m getting wrong.