The reason the game ran choppy was because:
1. It had slow RAM, because it was sharing the DDR of your system
2. It was a slow card with few pipelines and a low clock speed.
jervin, do you not get it? Even if it had 1GB of RAM that 6100 would still have been choppy, because it cannot process pixels fast enough because it has a poor pipeline. It is only partly to do with the RAM, and anyway, the core on that card is a lot slower than the RAM.
sung 13 - I would easily recommend a 6800GS, as they are faster than a 7600GT and will play Obvilion without being choppy
They are around $190.