There aren't many details about your Astec ATX250-3505 online. I think it only has 9A on the +12V line. Check the PSU label to confirm.
AGP cards need a minimum of 18A to run safely, so you'll need to upgrade the PSU before upgrading to a better card.
The SeaSonic 350W has an 80% efficiency rating and provides a combined total of 27A over two +12V lines. $43 from http://www.newegg.com/Product/Produc...82E16817151077
or shop around for cheaper deals.
You can get an AGP card with 2 display ports for around $30, going up to $100 for the better ones, depending on what you'll be using the computer for. See here for a list of AGP cards arranged in price order - http://www.newegg.com/Product/Produc...82E16817151077