I can get 733 no problem. Its the 1100 GDDR3 I cant get. I've played fallout 3 for 4 weeks on 733 but only 1050 RAM. I think its alot to do with cooling but it might also be that ATTools is quite old and that is the only thing I've tried, I was wndering what else is out there. When I put it too 1100 RAM it gets too many artifacts on screen and eventually crashes (~30secs).
Trying a different program wont help you, you've just reached the limit of what your card is capable of. Its a very respectable overclock for a 4850, anyways overclocking the memory another 50mhz wont give you much performance increase at all. Best to just leave it at 1050 and call it a day.