The ATI 9600 or the ATI/chip Sapphire (good card, just a little less expensive brand) will work in your system and be the maximum card to probably work correctly (and also new enough to use in an upgraded system if you wanted to in the near future). The nVidia 6600 would probably be the most for your system if you want to go the other route. There are many makers using the nVidia chips; some of them more economical yet not overclocked and plenty fast. We prefer ATI for the stability/dependability/performance that they have demonstrated over the years. However, the nVidia cards are quite fine now that they have improved upon them from the past and are fine in systems like yours also. You will find a never-ending debate on who is better....................to the majority of users, it doesn't make any difference..........................no more on this debate...................
Compatibility means how well a piece of hardware/ software app will work with other components or software...............Example:
The ATI/nVidia video card is "compatible" with your system;
Translation:
Either one will work "correctly" without problems in your system as according to the system specs.
Example:
Some games are "not" compatible with Windows XP:
This means some games (made for Win98/98SE/ME usually) may not run properly. There is a "compatibility" setting in WinXP that you can use to "see" if the app will run correctly or not. Sometimes they just won't.
Power Supply/Watts
All power supplies have ratings for the amount of juice they put into different circuits in the system. This is important as a computer is very susceptible to power fluctuations/supply. It is common practice to use a quality, bigger power supply in custom built systems to meet future upgrade potential. Proprietary systems (HP/Compaq/Dell/Gateway/etc.) don't allow for much that way. Here's a link to maybe help you better:
http://compreviews.about.com/od/cases/a/PSUWattage.htm