I seen guys "T" off the stock sending unit elbow to use both the stock and aftermarket gauges, or you could get rid of the stock sending unit all together.
As far as actual pressure thats different on cars and depending what pump you have. Mine is 40 psi at idle when its hot. 57psi at operating temp when driving. I believe I'm close to 70psi cold.
Mine is set up like LiquidGT suggests. I dislike having dead gauges in the dash.
As far as pressure mine runs about 50 at cold idle and 30-35 at hot idle. When I am driving it is usually around 60-70 PSI when hot. But the minimum that I would like to see is 20 psi at hot idle and then at least a 10 psi increase from there for each 1k rpm increase. So at idle 20 psi, 1700rpm 30 psi, 2700 rpm 40 psi and so and so forth.