So, if I were going to europe (which I am), I would want a phone with 900MHz GSM?

Some of the other Europeans will need to chime in here, but it seems that the older / state-run carriers tend to have service at 900 MHz and the newer carriers seem to have service at 1800 MHz. Current U.S. GSM service is at 1900 MHz (the "PCS" band, where you'll also find Sprint CDMA). New U.S. GSM service is at 850 MHz, which is the same old space where you find U.S. TDMA and AMPS analog service. The original AMPS channels get reassigned, one by one, to be either a bunch of TDMA/GSM channels, or a bunch of CDMA channels, which is how they "seamlessly" migrate from one standard to another.

But, back to your original question, you should figure out with whom your U.S. carrier has roaming agreements in Europe. If you've got a "tri-band" phone, then you'll be able to talk to any other European carrier, one way or another. Also (so I've heard but never verified), you may be able to buy pre-paid GSM SIM cards at the local convenience store. That would probably get you much cheaper local calling, but you'd then have a local phone number.