I'm in the middle of my IR training after just installing an IFD540 with a G5 HSI. One consideration is the number of airports with approaches that will be available to you with a GPS, and the number of airports with precision approaches that will be available with GPS w/WAAS. In other words, if you don't have a GPS, the number of airports available to you for IFR landing in the southwest drops dramatically.
Look on your favorite sectional map browser (vfrmaps, skyvector, whatever), and just browse the available approaches for various airports. Many airports only have GPS approaches, so if you don't have GPS, they're not an option as either a destination or an alternate. Also, if you don't have a DME, approaches requiring DME won't be available to you, either. An appropriate GPS will substitute for a DME, so it keeps your options broad. Look around on your usual or expected routes at the available approaches at airports along the way and see if it makes a difference to you.