I have an old computer which is AMD athlon processor and ASSUS motherboard and now I want to install a graphic card so which graphic card will suite to my configuration.

Four things must be taken into consideration:
1) The form factor of cards your Asus motherboard supports - PCI, AGP, or PCI-E?
2) The intended application? Cards for gaming or CAD software will be different from those for generic computer use.
3) What operating system are you using? The card must have drivers available for your OS.
4) What is your budget?

If you can provide more info as to motherboard model and the goals of your card purchase, specific recommendations can be made.