Contemplating gear change - help!

[mysweetlx]

I'm currently running 3.55's, but I'm thinking of changing to a steeper gear, maybe 3.73 or 4.10. Someone had mentioned to me that Ford makes a 3.90 too (I haven't been able to fin 3.90's online anywhere - where can they be purchased?). Which gear should I choose? My stang's a 5-speed driven mostly on the street, but I do go highways (80 mph driving) and hit the track about 3 times a year. Your help is needed - and appreciated. Thanks.