“Sneak Preview” does not sound encouraging. Which makes me think that the earliest we will see the new Mac Pro is likely to be October, possibly September, but not August.
And so, professionals needing to get work done now should go with an existing Mac Pro and its proven technology, e.g., a well-configured MPG Pro Workstation, minimizing the purchase of external peripherals ill-suited to the new model (e.g. eSATA-only, which would require a PCIe card which would in turn require a PCIe box to install it in). Ditto for Firewire and SAS. Think Thunderbolt, or USB3 for single drives.
I hear that 1866MHz DDR3 memory for the new Mac Pro is likely to cost no more than 5-10% more than existing 1600 MHz memory, so that’s a plus.