no. the 6870 doesnt have enough grunt to bottleneck a phenom 2 even if it is a low end cut down model.
chances are its your motherboard or memory thats reducing performance...
tighten the latencies on the ram and move everything off auto in bios to stock settings.
Unlock the two dormant cores in the Phenom II X2 555 Black Edition you have, and you will see an improvement. If both cores unlock and are stable, you will have a Phenom II X4 955 (B55). It will help the HD 6870 quite a bit. I had that processor in an earlier system with the lesser HD 6850 and, when unlocked to a 955, it made a huge difference in gaming. I can only assume like improvement in video rendering. Rendering needs more cores more than it needs a faster card. As for gaming, your HD 6870 should be good for nice frame rates at 1080p and lower with the unlocked processor.