I strongly suspect your AGP is the issue
Look in the Dxdiag, heading "Display Devices", fifth line from the bottom of that section, it says:
AGP Status: Not Available
This will be severly reducing the performance of your video card - the only reason it works at all is because you have a high end graphics card, and its inherent power has pulled you through. AGP enables much enhanced graphics performance. Below is a link to an article on it (a little out of date but still ok to get hold of the basics)
http://www.pcguide.com/ref/mbsys/buses/types/agp.htm
Its important you get hold of the basics, "read over" terminology you dont understand as such, and you will get the drift quick enough. Current standard is 8X AGP, which is backwards compatible with the older 4X standard. Your PC either has not got it enabled on the current bios(hence a quick check there needed), or you will need a bios and/or chipset driver update.
It's extremely important to performance that AGP is enabled. Without AGP enabled, everything may appear to be working just fine, but in reality only a fraction of the computer's true performance is being realized. Besides configuring the BIOS, the other necessary step to ensure AGP is enabled and running at optimal performance is to install the latest motherboard chipset drivers which enable AGP functionality for the computer.
Access your bios during bootup (usually by pressing the delete key or an F key - the screen will tell you), and look for the AGP line on one of the pages - if its there, change it to 128Mb. Save, Reboot and run Dxdiag - if still not showing, you need to find and install the latest chipset drivers for your motherboard. Look at your PC documentation it should tell you there, if not ring your original supplier for help in identifying the motherboard, they may even help provide uptodate chipset drivers. Load the drivers when found. There are various freebie utils on the Net which can help you identify the motherboard.
It now should work, however in rare cases you may need to update the bios, if your not used to the latter you need to get a friend to help who is aware of how to flash a bios.
It is important to do this - if necessary get a friend or workmate who is aware of this issue to help - you have a very good Video Card which is being crippled by no AGP access - it will make a significant difference to your PC performance
Regards
Zy