My suggestion is to go to tmobile store.
and borrow their 4g phone. and check the speed.
And compare with your maemo speed.

Then you will know whether it will make a difference or not.

Case 1:
Check the maemo speed at home (this will be your base)
Check the speed of maemo at the tmobile store (i assume this area will be covered by 4g otherwise find another store). compare the result.
Check the speed of 4g phone at tmobile store. compare the result.

Last casE:
Borrow the sim card of the 4g phone and test in your n900. then check the speed.

Then you will sure know whether its worth the $ to upgrade or not.