OK, I'll admit I don't know what gauges you have on you car from the factory, but maybe this will help a little:
oil pres and tach are always good.
FI: Boost and A/F ratio are a must, EGT might be helpful since it indicates incorrect A/F. It's my opinion that intake temp is important too.
Nitrous: A/F and EGT are both good. Not a gauge, but I would suggest that you have a window switch for any amount of nitrous.
Keep in mind none of these gauges do any good if you don't know what they are, what they should read, and how to tell when things are about to go south BEFORE damaging your engine. Figure this stuff out, and you'll have a good idea about what gauges you'll need.