AFAIK, you can just swap an XBee for an XBee Pro, because they're pin-compatible. But don't take my word for it!
If you're willing to do some soldering, I found a supplier for an XBee Shield Kit without the XBee. At $12, it's much cheaper than buying the preassembled shield + XBee from SparkFun or something. You can find it here: http://www.nkcelectronics.com/freeduino-arduino-xbee-shield-kit.html
If you're set on going Pro, you might want to wait until MaxStream releases a Series 2 version of the XBee Pro, which will include a new chipset with true mesh networking capabilities and a very similar command set (I think). Here's the MaxStream website: http://www.maxstream.net/
Are you going to use an XBee with your computer, too? You might be interested in this: http://tinyurl.com/2uo3fp
Hope that helps! 