is t mobile a 850mhz or 900mhz GSM network
 

Do you know if its one of these or something else

t-mobile is mainly 1900. And some 850, which I believe is mostly roaming off of AT&T. All T-Mobile phones sold now have at least 1900 and 850. So you want the full power of the network you need both bands (which are considered north american/asian bands vs euro bands).

If your curious, the 3G band is 1700 on t-mobile.

This is partially correct.

T-Mobile provides GSM service in the 1900 MHz band only. Roaming is not considered part of the network.

Further, the 1900 and 850 bands are not Asian bands, they are almost exclusively North/Central/South American bands. There are a few locations in Asia/Pacific that use 850 such as Guam and Australia, but they are the exception rather than the rule.