The Dell OptiPlex comes in four different sizes; the Ultra Small Form Factor (USFF), the Small Form Factor (SFF), the Desktop, and the Mini Tower (MT). Only the USF and SFF are small enough to make a reasonable sized All-In-One. Dell sells the following All-In-one kits through S&P (software and peripherals)/
Dell S&P P/N A0728637 – OptiPlex SFF + LCD monitor
Dell S&P P/N A1149268 – OptiPlex SFF + LCD monitor + Cover
Dell S&P P/N A0985007 – OptiPlex USFF + LCD monitor
These kits will work with any monitor that has a 75mm or 100mm VESA mounting pattern. This includes all currently available Dell monitors. The kit has been tested with monitors as large as 30″ with no adverse effects on stability or usability.