Indeed, you beat me to the punch!
1152x870 tops out at only 16bit in the 2MB VRAM limited 840AV. For everyday use or gaming, nothing beats onboard video, but that's not a NuBus Card's intended use case. Thunder Cards are the bomb for displaying big graphics in 24bit, there's nothing like 'em. I lucked into the Radius Thunder IV 1600 as it was mistakenly listed as a much lesser card.
Happened across an auction for a Thunder II 1360 - anybody have any data on where this card fits on the spectrum
SuperMac Thunder II GX 1360? I'd definitely use that on a daily driver for running a modern 17" LCD in native 1280x1024 at 24bit. That's second fiddle only to a GX 1600 in 24bit on a native 1600x1200 panel.
Run a second display off the 840AV video for gaming and such for the best of both worlds or save your money if the onboard video will suit your needs.