For back you can add pullups/chinups (various grips)…these are some of the best exercises to develop your back. Also look here for more back exercises: http://www.exrx.net/Lists/ExList/BackWt.html
Also, don’t get too carried away with doing a ton of sets, your arms get pretty taxed when doing chest and back…anything more than 2 exercises may be overtraining…if you break it up you may be able to hande 3 exercises…but if you overtrain them they will not grow at all.
That site is a great site to reference…tons of good info on there. Also for the “300 lb club” once you can press 225 for a clean 10 reps you should be in the ballpark. Once you get there you will want to push 3 plates on each side…then you will want more…it never ends!!!