I recently purchased a 1999 mustang gt with a 5spd.  I was wanting to change the gear ratios but i am not sure if i want to go with 3.55 or 3.73 gears.  How much difference is there between these two ratios performance wise and also how big of difference will it change my 0-60 and 1/4 mile time?  Also how much is my gas mileage going to decrease from stock with these gears?  Thanks  

I was going to post up a link to an online gear calculator but when I googled it, 1000s of them popped up.  You can probably find the exact answer there, but as far as opinion and experience go, I'll give you my answers.
Going from a 3.27 gear to a 3.73 is a fairly big jump and you'll notice the acceleration difference.  I've run 373.s and 4.10s in the past and found that 3.73 is the perfect blend for me with street driving and track driving.  4.10s were a little too much.  I'm thinking if you put 3.55s in the car you may decide later that it was not enough.  I'd also guess in 5th gear you'd probably see about a 200rpm difference between the two.  With stock gears and you tires size, you are probably close to 2000rpm at 65mph in 5th.  If you ran 3.55s you'd probably be close to 2200rpm, and with 3.73s you'd be at 2400rpm.
Gas mileage will go down as you'll like the sensation of added acceleration and you'll drive more enthusiastically thus getting worse mileage.  To pick a number that would depend on your foot, but if you drove the same as you are now (all things being equal), you may loose 1mpg on the freeway cruise, and maybe 1.5 in the city.

Something else to think about would be that if your car has more than 80K on it, when you swap gears, I'd also get a better differential in there.  The sotcker is probably shot.  You can probably have it rebuilt if you choose as well.  

I went from stock to 3.73. I LOVE IT! rpm on highway is at 2600 at 80mph, It is perfect for around town, and does well for autocross as well. My hwy gas milage was 25 on a road trip doing 80-85 average.  

Ok thanks, I am planning on going with 3.73s, thanks for the input.