I went to 4ghz from stock, havent tried below that, but i940 (same settings would apply for i920)
For 4ghz (20x200) 12hrs prime stable, QPI 36, uncore 17x, mem8x (uncore has to be at least 2x mem or no post)
Vcore 1.356 bios, LLC enabled, qpi/vtt 1.31, DRAM volts 1.64, 8,8,8 24 1600 (stock is 1600 at 1.65), qpi pll 1.14 (1 notch over stock 1.1)
rest stock volts. I have 1600 rated ram. If you have 1333 rated you may have same settings but 6x mem multi.
idle temps 38C, load 70C on water, GTZ/PA 120.3. People on air with aftermarket coolers are getting load temps in 80's at equiv settings.
For 4.2 (21x200) 12hrs prime stable, exact same settings a 4.0, except vcore increase to 1.41 bios, vtt/qpi 1.33, and IOH, qpi Pll and cpu pll volts all 1 notch over stock.
idle 39C, load 75C temps, at 25C ambient.
Regarding mem speed, some have 1333 DDR3 and using mem ratio 6x, uncore 12+, those using 1600 DDR3 mem ratio 8x with uncore 16+. 2000 rated needs very high qpi and way over intel maxes. I have run my 1600 rated at 1333 speeds, 1600, and 2000 (as have others). Makes no difference in any real world application, game or any benchmark like pcmark5, spi, etc, only makes a difference in benchmarks that test memory bandwidth specifically, ie like everest, sandra. But given minimal price difference between 1333 and 1600, I would go with 1600 rated, gives the most flexibility for settings.