The i7-930 and 920 are seemingly identical apart from their clock frequencies. Each has four cores and eight threads, 8MB L3 cache, 4.8 GT/s QPI, a triple-channel memory controller, and a 130W TDP. The extra six bucks gets you a 2.80GHz clock speed with a 3.06GHz Turbo Boost, instead of 2.66GHz and 2.93GHz. Given the minor differences, Intel may phase out the i7-920 altogether, but that is currently unknown.
It's worth noting that the Core i7-860 is arguably a better buy than either 9xx chip. For $279.99 it offers a 2.80GHz base speed with a 3.46GHz Turbo Boost, a 95W TDP, and it runs on a cheaper platform (LGA1156 versus LGA1366).