I had 4.30's in my 2003 Mach 1 (manual with .62 overdrive) and have 4.10's in my 2007 GT (automatic with .71 overdrive). The overdrive helped sway me to 4.30's in the Mach 1. Not enough overdrive in the 05-07 Autos, while the 05-07 Manuals have a .68 overdrive so they can take a little more gear.
More importantly, what are you going to do with the car? I you run a shorter tire than stock, install the 4.30's (or 4.56's), do some popular bolt on's then head to the dragstrip you might just run out of 4th gear by the end of the 1/4 mile. I calculated how much gear I would need to hit redline in drive (4th) at around 120 mph (planning ahead for mods) and I came up with 4.10's as the right gears for my application. You should do the same math, or perhaps figure out cruising rpm's on the freeway if that's important to you.
Of course, you could always run a taller tire to gain back some mph with a steeper gear.
Both my 03 Mach 1 an the 07 GT only suffered about 1/2 to 1 mpg drop versus stock gearing. Not really a big deal considering how much better the car performs! DO NOT FEAR THE GEAR!