The Core i7-920 is a great choice for systems coupled with multiple graphics cards in an SLI or CrossFire configuration.
The motherboards and DDR3 RAM that the i7 architecture requires will bring the total platform cost higher than other systems, but the resulting performance should be worth the purchase price.
While the Core i5 performs similarly, there are a few applications and games that can take advantage of the Core i7 900-series' Hyper-Threading and triple-channel memory features, so spending the extra money on the Core i7-920 can pay off, particularly if you plan to overclock.
In addition, LGA 1156-based Core i5 and Core i7 processors are limited to 16 PCIe 2.0 lanes, but the LGA 1366-based Core i7-900s do not share this limitation, since they get their PCI Express connectivity from the X58 chipset. This makes the LGA 1366 Core i7 processors a good choice for CrossFire or SLI configurations with more than two graphics cards.