A lot of peopel on this forum just jump on the ban-wagon and just go all out and get the 4.10s. After having 4.10s in my 98GT (auto) and now having stock gears in my 02 GT and after doing a LOT of research and RPM calculators in comparison to my driving style, here is my recommendations:
(all below recomendations are for MANUAL cars, i personally wont recommend gears in autos, just my thing...with autos + gears your tranny will heat up fast on long trips. Might need a stall and recalibrate your Overdrive)
3.55s -- much better over stock, you really should ditch the stock gears ASAP and at least go with these, for a little extra kick. Great for highway.
3.73s -- If your car is supercharged, or will be in the near future. Also a good gear for constant highway driving/commuting, but still want some fun.
3.90s -- im biased on these

personally i feel go 3.73s or go all out with the 4.10s, dont get this bastard "happy medium" gear

But they might be somethign to consider if you are really torn between the 3.73s and 4.10s
4.10s -- great for the track and if you're a "light to light" racer

Definately will feel an improvement on a manual car, but you'll find they rev kind of high when cruising on the highway. NOT recommended for supercharged/poweradded cars
4.30s -- Great gears for N/A GTs and Cobras for the track. I really wouldn't recomend these on a daily driver car, and the highway would be a bit excesive...
I personally will be going with 3.73s on my GT because i highway commute everywhere pretty much non-stop. And 4.10s would be too much for me. It all comes down to personal opinion. But i would HIGHLY recommend you get one of those links to an RPM calculator...and start takign notice of how you drive around town and on the highway, and what speeds and RPMs you travel at. Then compare them to the gear calculator and see what is best for you