T-mobile $30 pre-paid plan on upcoming Windows 8 phones

I am writing to get some information on T-mobile's plans and phones. I am tech savy EXCEPT where it comes to cell phones. I want to explain what I want to do and please let me know if this will work.
Ok, I have been on data plans with T-mobile and Sprint in the past. I NEVER talk on the phone, text A LOT, and don't use a lot of data but want/need data for directions, fantasy football, etc.
T-mobile's prepaid plans look good to me. The plan I am thinking about getting is the $30/month (100 mins talk, unlimited text & data). My question is this:
If I buy a T-mobile Sim card or Monthly 4G prepaid cellphone from a store, Can I just activate that phone and then put that sim card into one of the new unlocked GSM Windows 8 phones when they come out and everything will work? I've read other forums and it seems that as long as a phone is a t-mobile phone OR unlocked and operates on the same band as t-mobile, the phone should work. Is this correct?

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You should only need to purchase the T-Mobile prepaid SIM card, not a T-Mobile phone. It should work if the Windows 8 phone is the same frequency (band) as T-Mobile, like you said.