Two main reasons the i7-860 is better than the Q9550:
1 - Hyperthreading.
2 - The big one: Turbo Boost
While both are quad-core chips and have roughly the same base frequency, the architecture is VERY different.
The simple version:
Hyperthreading allows the OS to see 8 "virtual" cores on the i7-860. This allows the processor and OS to handle processing tasks even more efficiently. Turbo Bost allows the i7 series chips to automatically overclock themselves depending on the load.
Sure. No problem. Though, it isn't so much the Interconnect Bus speeds but the whole Bus architecture that has changed from the older Core 2 design.
With the Core i series chips Intel has moved the memory controller on-die and abandoned the Front Side Bus for a more robust component interconnect system, QPI - or QuickPath Interconnect, in their higher end chips (Bloomfield and upcoming Gulftown i7-9xx series). The QPI implementation is similar to the Hypertransport that AMD transitioned to several years ago.
In the low-end and mid-range chips Intel essentially moved the entire Northbridge on-die (memory controller, PCIe) and uses DMI (Direct Media Interface) for communication between the CPU and the I/O hub (hard drive, USB, etc...). This is what the i7-860 uses. Because of this, the CPU has direct access to the memory controller and the PCIe lanes without having to communicate over the slower FSB.
Personally, I would not waste the extra $50 or more to get a socket 1156 board
with more than a single PCI-E slot, especially one that runs the second slot at 4x.
I would get a nice $100 board with a single PCI-E slot.