About 2500 I think. The car will feel more on edge, like it wants to go. I personally like the way the car acts with stock gears, but you can't beat the performance of the 4.10s.
Go to www.smokemup.com You have to register, so it will be one more password for you to remember. Anyway, it has a bunch of different calculators on there that will help you figure out RPM for your gear changes.
I am doing about 2600 RPM's at 75-80 MPH with 4:10's.
I also avg 26.4 MPG on the hwy at those speeds.
Don't even bother going lower unless you do a lot of high speed hwy driving
4.10's are great - BUT you will NEED a MMC shaft and a chip. Be 100% sure to get the 3-4 WOT shift at 4800rpm in 3rd feature removed with the chip (applies to 96 up Marks only). You also need a new VSS sensor.