Easy enough. Your system shows 480MB of RAM because the onboard video SHARES the system memory. Its set for 32MB. Add 480 and 32. 512MB.
Go into your BIOS and go to the PCI/Plug N Play setup page. Find the setting that says Primary Graphics Adapter. Selecting AGP allows you to use the onboard or the 4x AGP slot. You can also set the amount of memory the onboard uses, etc. Allocate IRQ to PCI VGA should say NO. Save and exit.
Make sure the video card is fully seated in the AGP slot if its not reading. AGP cards can be tricky that way. It may LOOK seated, but if you see even the slightest amount of metal on the card contacts where it slides into the slot, try wiggling it.