i dont know which graphic card supports my mother board i have brought the system 7 years ago and its slot is agp i think and my ram is ddr how do i know which graphic card of nvedia i should install to my pc
-google the model of your mobo to make sure of the expansion slot type;
-google some nvidia cards that fit that slot.
-ensure that the PSU can support that graphics card. This is important for the stability of the system.
It's a matter of what type of graphic card does the mobo support, and if the PSU will give it enough power under load.
nVidia does not manufacture graphics cards for end users, just chips used by other manufacturers. If you're looking for an AGP card, your choices will be rather limited anywhere but eBay. The type of RAM used in your system has no bearing on what type of graphics card is supported.
AGP cards based on the Radeon4000 can be a real pain. Catalyst drivers don't really support that configuration and all the AGP cards with these GPUs are the result of GART tinkering by the card vendor. I bought two of these to try and breathe new life into some old P4 systems I had for HTPC use but got tired of pulling my hair out to get anything resembling a stable set of drivers running.
In 2012, spending any money on AGP just isn't a good idea. If you're lucky, you might find a good AGP card (GF7800 or X1600) on eBay or Craigslist but even then, it's going to be 5-6 years old and it's hard to find one for a fair price since most of the people who have them know there's someone out there who just has to have one and will pay stupid money to get it.