There are AGP versions of the HD 4670; like the HIS Ice Q.
Not sure what the OP means by "512 AGP support". Is that the aperture? I believe the way it works is if the video card is running low on memory, the system RAM will be used to store textures. Basically, the more VRAM the video card has the lower the aperture should be set to.
Back when I had an AGP Radeon X1900XT 512MB, I set the aperture to either 64MB or 128MB.
So the lower setting did not have a negative impact set to a lower number than the video cards higher onboard ram than apparently supported by the motherboard?