Jason,
Rather than buying a SIM in Europe, you could also consider one of the "travel SIM" firms such as Roam Simple, Call In Europe, Cellular Abroad, Mobal or Telestial (or others).
While you could purchase a SIM in Europe, keep in mind that while it will provide inexpensive rates in the country of purchase, when you travel to other countries you'll be "roaming" and the rates will increase. Also, it's likely that you'd be using a pre-paid plan so you'd need to top-up when your minutes are depleted.
The SIMS provided by the above firms have consistent rates regardless of which country you're in (except for some countries in eastern Europe), and some of their plans are post-paid meaning your credit card will be billed at the end of the month (when they receive the summary of charges from the cell networks). Topping-up won't be a concern.
I would highly recommend checking your T-Mobile phone to ensure that it's a quad band model. I ran into some tourists from the U.S. last year that were mystified that their "GSM Phone" wouldn't work in Italy. I checked the phone and found out that it was a dual-band model with only the two North American frequencies.
It would also be a good idea to check the Phone Charger to ensure it's designed for "world operation".
Cheers!