I see that your graphics slot is AGP Pro 1.5V.
Most new cards these days are PCI-Express, which is incompatible with both AGP and PCI.
You may have a little trouble finding a DX11 card because, as far as I know, none are on the market yet. I'm not sure when the ATI 58xx series will be on the market, but it may not be until Win7 GA (22 October). (Only Win7 supports DX11.)
I'm not familiar with AGP Pro, but apparently it is compatible with regular AGP cards. (The AGP Pro slot is longer than regular AGP, so AGP Pro cards can't fit in regular AGP slots.)
I suggest that you visit
www.newegg.com, whether you are able (or willing) to do business with them or not. Go to the desktop video card area, and go into the power search. Check the AGP and AGP Pro interfaces, and the DirectX 10 and 10.1 boxes. (DirectX 9.0c would serve for Win7, but Newegg doesn't provide that level of detail. Apparently some of the older DirectX 9 cards don't support the shaders needed to run Aero Glass.) You'll get a variety of ATI (AMD) cards. The most up-to-date AGP cards use the HD4670 chip. Looks like you may need a spare Molex connector to provide additional power to the card.
I have the impression that getting the ATI Catalyst drivers to play nice with AGP cards may be more trouble than with PCI-E cards