I had the same issue when I first tried it, the only setting I had changed was the resolution from 800x600@32 to 1024x768@32.. It would crash as soon as I entered a game. So I deleted it and unpackaged it again, not changing anything and it worked fine for a while.
Now all of a sudden without changing anything it started crashing on joining games again!
Also, I'm not sure if this was coincidence or not, but on every server where I started issuing orders to bots (telling them what to drop and where mostly) they crashed a short while later and this happened on 3 different servers in a row..
I also have the G4 1.25 dual with 1GB ram and the base radeon 9000.