I have a HD4850 512mb aswell, also a AMD Phenom X3 and 2GB 667 ram.
The demo ran flawlessly, 100% smooth. I installed the game today, everything high, natvie res at 1680x1050 and there was lag though still very playable. When bowling or batting everything is smooth but as you said, when there is a pan around the ground it stutters. So I knocked the res down to 1440x900 and its much better. Still the slight jitter every now and then though.
The demo ran flawlessly, 100% smooth. I installed the game today, everything high, natvie res at 1680x1050 and there was lag though still very playable. When bowling or batting everything is smooth but as you said, when there is a pan around the ground it stutters. So I knocked the res down to 1440x900 and its much better. Still the slight jitter every now and then though.