[Proj] X,Y UTM distance calculation (compare with latlong WGS84)

[oon at informatika.org]

     Dear All,

I want to ask a question about distance calculation of two points in 
X,Y (UTM).
Is it ok if I get the distance of two points in X,Y (UTM) only with distance
calculation of cartesian coordinate (pythagoras method)?
The error getting larger (or getting huge) if I calculate the distance 
with the
points crossing the UTM zone. (I have tried for point with (1) different zone,
I don't know what will be happened if the points over more than 1 zone??)

Is it any other method to get distance calculation of cartesian 
coordinates X,Y (UTM), so the result of the calculation have accuracy 
like distance calculation
of WGS84?
(It's ok if the error about hundred meters)

thanks for your help..

-- 
regards,
Oon Arfiandwi
Software Engineer
Bandung, Indonesia.