If the price is right, go for it. For just basic operations, there is no extra cables you will need (usually).
Your PSU will need to have a special connector for PCIe graphics cards. If it doesn't have one, you can buy an adapter.
Does your monitor support DVI input or is it analog? The X1900 only has two DVI outputs I believe, but again you can easily buy an adapter.
You can download the drivers for it on ATI's website.
It also has an Svid out port, and you can buy an Svid cable if you want to use that feature.
See what some others say (especially those who own the card).