Just upgraded from a Driving Force to a G25 myself and the 900 degree steering is absolutely worth having. I haven't used the Driving Force Pro (also 900 degrees) so I can't tell you if the G25 is worth the additional cost over a DFP when playing GT4, as you can't take advantage of the clutch and gearbox set, but I also play some computer racing games that can take advantage of the extra controls, and for these the G25 is fabulous. If GT5 takes advantage of it, then I'll be very happy. I just wouldn't want to go back to a simpler system now.
I don't know if any of these wheels are X-box compatible.
EDIT: Oh, and for a G25 (and I guess DFP) make sure your cockpit is nice and sturdy, and your driving position is particularly good. I could get away with odd driving positions with the Driving Force (plank of wood sitting on two cardboard boxes did fine) but the G25 has made it obvious that I need to invest in a proper dedicated cockpit to get the most from the wheel.