Question : Problem: dual link dvi with Dell 5100 mp

Is there any point in buying dual link dvi-d cable for the 5100 mp? In other words, are any of the machines digital inputs capable of using dual link signal? It has an M1-DA input and One HDMI input. The hdmi imput is 19 pin type a

Answer : Problem: dual link dvi with Dell 5100 mp

The quality of DVI cable that you get can make a difference.  I once bought a cheap 30-ft DVI cable, thinking digital means you don't have to worry about how it works, because it either works or it doesn't.  To my surprise, the cheap cable produced green "sparklies" in dark areas of my screen, an indication that the cable was acting as an antenna and picking up interference.  That said, you shouldn't have to pay an arm and a leg for a good quality cable, so going with a reputable store will probably save you money.

Dual link does not mean you are getting a better cable.  From http://www.datapro.net/techinfo/dvi_info.html:
"The Digital formats are available in DVI-D Single-Link and Dual-Link as well as DVI-I Single-Link and Dual-Link format connectors. These DVI cables send information using a digital information format called TMDS (transition minimized differential signaling). Single link cables use one TMDS 165Mhz transmitter, while dual links use two. The dual link DVI pins effectively double the power of transmission and provide an increase of speed and signal quality; i.e. a DVI single link 60-Hz LCD can display a resolution of 1920 x 1200, while a DVI dual link can display a resolution of 2560 x 1600."