As indicated above, the question is easier to answer if we know the Processor / MoBo combo. Also important is the quality of the PSU because as your GFX cards pull amps, voltages tend to slip a but and how far you can OC much depends on how stable that voltage is. Tier 3 PSU's will meet the ATX spec of +/- 5% ...... Tier 2 (Corsair TX, Antec EA / TP) will generally get ya 2-3%, Tier 1 (Antec SG / CP, Corsair AX / HX, XFX Black, Seasonic X) will get you 1%.
The Power Regulation of your MoBo also comes into play. Enthusiast OC boards like the Rampage series from Asus invest a lot of their component dollars on this part of the design.
Some processors OC better than others....in general, the newer the better. Sandy Bridge processors can get up to 4.6 GHz on the stock cooler. There are many web sites which keep databases on cooler performance but it is important to know what CPU socket is used on their test bed as CPU's of different types have entirely different heat signatures. For example, frostytech.com uses the 775 socket CPU's making the test data irrelevant to 1366 or 1155 CPUs. Performance even varies among processors as you will see minor performance differences between even the 920, 930, 950, 960 as the 1-2-3 positions on the 920, might switch to say 3-1-2 on the 950.