If your motherboard has a pcie-X16 slot, then any graphics card that uses a pci-e X16 slot(most do) will fit.
What might not fit is the physical dimensions if you have a compact case or a slim case. Get the dimensions of your prospective card and measure the space in your case.
The other concern is the power supply. A 6850 needs a 450w psu with a 6 pin pci-e auxiliary power connector