From what I've read, about 20-30g of protein at a time can be used for muscle protein synthesis, so more than that in a meal is somewhat wasted (at least, from the standpoint of repairing or building muscle). That seems to be the recommended post-workout dosage for strength athletes, I would think endurance athletes might not need as much but I don't think there would be any real downside to consuming that much.
Actually, the jury's still out on how much is too much. There have been some studies that show it could be stressful to the kidneys in some way but nothing even close to being conclusive at this point. Nevertheless, the above seem reasonable. And maybe 10gms. on either side for a man.
The idea behind glutamine intake is that of muscle sparing: if its readily available in circulation it would spare your body's use of this most abundant amino acid. Creatine, on the other hand, draws water into the cell which theoretically should aid with protein synthesis and add a bit more energy for one or two more reps in the weight room. It works well for some, while others (me included) report just getting bloated.
For the record, no one knows what the limit is for protein absorption, but if you're currently taking protein isolates, you're already getting the very best of everything you need.