In case you've grown attached to your old rig, you'll be happy to hear
that Gigabyte is now offering an AGP Radeon HD 4650 card,
Whether you're upgrading, or just want to use an old PC as an HTPC, an
AGP HD 4650 is a pretty good choice, as it's not nearly as power hungry
as previous AGP cards. With a power consumption of just 48W, you
probably won't have to upgrade your PSU to enjoy HD and a few more fps
in games.
Mind you, the HD 4650 is not the fastest AGP card around, vendors have
been selling HD 3850 cards for a while now. In spite of that, we're
guessing it will do just fine on an older system, as it's more likely
the CPU will be the bottleneck, and not the HD 4650.
You can find the product page
here.