Don't forget that it's actually just $80 you'll spend for the two $50 refill cards if you buy them this week at Target.
Get those first even if you haven't picked out a phone yet. Once bought they are activated and you have up to 90 days IIRC before calling that fastcard 800# for the real T-Mobile pin.
BTW, if you had a credit card set up for auto top-up on Virgin, you could have selected the $15 every 90 days option.
And another thing... check T-Mobile's To Go coverage in your local area to be sure you'll be getting service and the signal strength. Their site has a street level map thingy.