I want to buy an agp card 512 mb , which company card should i buy ati radeon or nvidia?

Both video cards are good in their own way. ATI boasts their cards with shoving a whole bunch of RAM into their cards. But the chips are okay. NVIDIA focuses more on the chips, and has average amounts of RAM. Because of this, ATI is used more for HD movies and NVIDIAs are used more for computer gaming. You could p with either one, but each one has a different edge in what they work best with.