Anoobis
Splendid
Machspeed Tech.
KT880-8237
Socket 462, Athlon XP, right?
My guess is that you're running one of the Viper V881DAS flavors in which case the VIA Hyperion Pro Driver Package should be all you need as it includes the AGP component.
It might have something to do with what advcomp2019 noted about the PCI-E bridge.
Generically speaking, the card's architecture is natively PCI-E. Somebody pipe in if I'm wrong on this. ATI and the manufacturer modified it to AGP and installed a PCI-E bridge to AGP chip so the PCI-E architecture could communicate through the AGP bus system. Could it be that because the card is natively PCI-E, the system doesn't recognize it as an AGP card? Perhaps the card is working correctly and that's just how the system views it.
After you install the ATI driver, what does it say under adapter type in the Display Properties?
Can you run any games after you've installed the ATI driver?
You might want to take it to a shop that has another AGP X1600Pro and see it it recognizes their card the same way. If it doesn't then I'll go back to my first guess that there is something wrong with the card and you should return it for an exchange or refund.