That is a pretty old PC. Compared to a Core i3-2100, the Pentium D 3.0GHz is about 30% - 35% slower. It will likely bottleneck the HD 6850.
Additionally, that old PC only comes with a 200w power supply (probably 150w on the 12v rail). The HD 6850 alone uses a max of 120w on the 12v rail. That does not leave enough power on the 12v rail to power things like the CPU. You would need to upgrade to at least a 400w good quality power supply.