What is the highest resolution I can use with OS X 10.7.5 and DELL 30inch monitor?
OSX lion doesn't limit screen resolution, screen resolution is only limited by the graphics ability of the computer you're using and the screen res of the monitor you're using. If you're running lion then you have an intel chip mac. Older intel macs (pre 2009) with shared GMA video cards supported up to 1920×1200, however pretty much anything post 2009 will support up to 2560×1600 (which is what your Dell 30 inch monitor will be if its the current 3007WFP model)