Ears,
Yes, 24-bit/192kHZ is the current top of the bit/sampling rate heap. I think there are some higher sampling rates for a few esoteric units, but most high-upsamplers are 192kHz.
For the Ah! 4000, I think the upgraded upsampler is a replacement of the 24/96kHz board that comes in the stock Marantz unit that Ah! modifies to create their model.
Not sure the price of the Ah!, but if you are planning to spend more than about $800 for a used unit, the Quad 99 CDP cd player is excellent. The Quad has an excellent transport, excellent 24/192 DAC and it has 6 additional inputs so you can use its internal DAC to improve a DVD player or other digital component. Additionally, the Quad has an excellent built-in, remote controllable volume control. You can connect it directly to a power amp saving dollars normally spent on a pre-amp or pre-processor. The Quad is excellent at its new and used price. It is an especially good deal if you have an all digital system and can get rid of an expensive pre-amp, using the Quad's remote to control the volume.
Enjoy,
TIC