The quest of a video card for Core Image glory

Hi-

Most readily available 4x AGP for your QS is the ATI Radeon 9800 Pro Mac. Supports Core Image, and Quartz Extreme, and has one of the fastest GPU's available for 4x AGP.
These graphics cards are cards that are or have been offered by Apple, that support Core Image.
There are 4x AGP PC cards that can be converted, "flashed", that also support Core Image. Typically, the models will be those that Apple has offered, but ATI or Nvidia doesn't produce anymore. Examples would be the ATI Radeon 9800XT, or the Nvidia Geforce 6600. A real fast flashed card that can be used in the 4x AGP machines, that also supports Core Image, is the Geforce 7800GS.

The Radeon 9800 Pro Mac 256mb does support Core Image (I have one). Also, the ATI Radeon 9800 Pro 128mb, both Mac and the PC version, support Core Image (I have a flashed PC version, also).
It's not a "magic" thing, Core Image support- it's a function of the CPU's power or the ability of the GPU to support Open GL Shaders. Any programmable GPU that supports the required Open GL Shading Language (GLSL) commands is capable of rendering Core Image. If Open GL Shading Language (GLSL) is supported by a card, then it can be inferred that Core Image is supported. To support GLSL, a GPU must be programmable with regard to the shaders.
So, Yes, there are many "unadvertised" cards out there, that can support Core Image.
A "feature" of Core Image, is that if the GPU isn't capable of supporting Core Image, the process automatically falls back on the CPU. Core Image and the GPU.

Though not directly stating "Core Image Supported", the specs of the 9800 include "Advanced programmable vertex and pixel shaders in hardware", which is a requirement of Core Image support.

I'll be traveling to the USA in a few weeks, and I'd like to get ahold of one of those cards there.

I've never seen a retail store with Mac compatible video cards.