I have the oil presure and water temp gauges but I have room for one more, what would be best? I realy am not sure what the vacume gague will tel me, if it is important to know or not. I was also considdering an air/fuel gauge. Any sugestions?

Voltage. If you plan on running a pretty good ICE set-up.

if your not running f/i get a voltage gauge i guess...I am running 6 gauges-Fuel Pressure, Oil pressure, trans temp, water temp, boost, and oil temp. I gotta still get a either a A/F gauge or a AEM UEGO.