p4inz - what year is your car? Odds are you have 3.27's now you may not see a big pick up going from a 3.27 to a 3.73. I have 4.10's still get 24mpg on the hwy.
Ok i was going to say Nitrous. I suggest you look at all the companies they each have their supporters. I like NX customer support is above and beyond any of the others IMO. You didn't mention if you were looking at a wet kit or dry or powerlevel. I prefer a wet kit and I stick with the 100hp nx jetting. With that my car makes over 400rwhp and over 500ftlbs on a superflow dyno.
Now for things to consider with your nitrous.
1. Bottle heater - not really a saftey item but you will need the bottle pressured up to whatever psi you choose to run at.
2. Colder Plugs - I suggest tr6's they are one step colder
3. SVT Focus pump - I ran without one and I'm lucky I strongly suggest getting one.
4. DYNO TUNE - I was not a big believer in dyno tunes, that was till I had my car strapped down and the pulls started the lean spike coming in was AWFUL I am lucky I didn't blew it up racing all last year without one.
5. Blow down tube - Some tracks require one for any car some don't
6. Fire jacket - NHRA rules say any car with an aftermarket power adder must wear one.
7. Window switch - I ran without one and I use one now it is much easier I don't have to think about when I mat the gas I just have to drive.
8. FPSS - some swear by them and they are a good idea, also suggest not spraying with less than 1/4 tank I don't spray with less than 1/2 a tank.