Multiple studies have found that using two or more screens can substantially improve performance (University of Utah, 2008; Colvin et al., 2004; Czerwinski et al., 2003).
Placing the monitor too low can increase neck strain. Placing it too high (the top should not be above eye height) can also cause problems. Even more important may be the adjustability of the depth, allowing for optimal viewing distance all the time. This helps you absorb information and causes the least eye strain. Rather than work in the wrong position, this optimal monitor position is much more comfortable and increases productivity by 10% (Sommerich et al., 1998).