25 turbo is the single best bang for buck upgrade you can do to the RB20. As for the gearbox, not sure about the auto's, I'm going to put a 25 engine in front of my 20 box and see how long it lasts. My mates 25/30 commodore was running a 20 box for about 6 months (because it had an RB20 in it before the 25/30) making 490rwhp, it eventually snapped 3rd gear in a big way.. loaded it up in 3rd from low revs and when it hit boost it went BANG! Depends on driving style I guess, I tend to abuse my cars a bit more than him