I'm interested in this too. /forums/images/graemlins/ears.gif
From what liitle digging I've done, it seems racecars use the 1.75" and thats what many have suggested. However, given the extra weight of our trucks I think I'm gonna opt for the 2 x .125" tube, at least for the main hoops. I can get 20' of the 2" for around $50.
Don't quote me on any of this as I have only just begun looking into build a cage... /forums/images/graemlins/deal.gifstill much more research to do