I have to disagree with those who say an AGP card is not worth it. If you've got a nice rig that uses AGP then go for it, but only if it's a NICE rig. Otherwise, you may as well build a new computer, since an ancient PC wouldn't even be able to run these cards at all.
It really is a joyful experience to squeeze just that much more life out of an old rig, and AGP can still haul if you need it to. If you're going to be playing games in the range of these cards, and have a processor, Motherboard, and PSU to support them, it's probably a bit cheaper to buy a $120 card than to build a $700 system.
You gotta be sure you can run your new card though, since the more powerful AGP cards have some pretty high standards in relation to AGP systems, as their age.