No worries, mate, the latest and greatest will still run DX9, although they'll often take a bit of a performance hit ( my Win 7/HD6950 will run the original Unreal, and that's a DX5 title!).
More detailed specs would be helpful, though-some older PCI-E 1.0/1.0a motherboards have issues with PCI-E 2.1 cards.
BUT, dependng on what one is currently installed, you may need to replace the powersupply to cope with the replacement card.