First of all, welcome to the funny farm!!!
What year stang do you have? I will assume that it came with 3.27's stock. When asking questions in the future, be sure to tell us some more info about the car.
Gears? What you are doing is changing the final ratio in the differential. As the gear ratio number gets higher, your car will run at higher rpms to achieve the same speed as compared to stock. The car will also be quicker. Going from 3.27's to 4.10's will feel like adding about 40 hp and you will knock about .5 seconds off the 1/4...maybe more...depends on too many other factors like tires and other mods.
One thing to keep in mind when you start to mod your car are your future plans. For example, if you plan on adding a turbo or supercharger (sc) soon, 4.10's will be too much gear. But for an n/a car, 4.10's are a great choice.
You will NOT loose any performance, but you will loose about 2-3 hp as the 4.10's make a little more friction....but don't wory about this at all.
If you have an automatic, you will need to calibrate your speedo for sure with the new gears to avoid ruining your transmission. You can do this with a speed cal ($100) or a chip/programmer ($200 - $400). A chip/programmer will do a few more things for you than just a speed cal. With an auto, get a chip as soon as you can as it will firm up your shifts and rev the car higher, thus more power.
With a 5 speed, you don't have to calibrate the speedo, but I would still recomend doing it.
Either way, I would go with a programmer as it will grow with your car as you make mods (this is a whole other subject).
Bottom line, 4.10's are a great first mod or a last mod. You really can't go wrong. just be sure to have them installed by a pro.