NuBus handles 1600x1200@24bit and 2048x1536 is only a 64% increase in pixels. I don't see where that might pose a problem for PCI, even in its earliest iteration on the Mac.
Well, remember, the bus speed of the slot the card is in has basically nothing to do with how capable the video card can be; unless you're doing something like advanced 3D work where textures are grabbed straight from system RAM the refresh for the monitor comes from the RAM on the card, not across the bus. (IE, the limiting factor is how fast the memory subsystem on the card is, the speed of the RAMDAC, etc.) Where bus speed comes into play is actually updating the display; again, for instance, if you had a 2048x1535 card sitting on a PCI bus *if* you had a sufficiently fast disk/RAM/CPU subsystem to generate bitmap content at an essentially infinite rate you'd still only be able to shove about 10 frames per second into the card's VRAM. (This presumes that the RAM subsystem on the card is capable of sustaining the roughly 700MB/second read rate it needs to update the monitor at, say, 60fps *while* it receives data at 133MB/s, of course.)
To apply the same idea to your Nubus card, at about 4.5MB a frame a 1600x1200 Nubus card is only going to be able to update about 4 frames a second. (Again, this is shoving frames directly from main memory into the framebuffer, and assuming that the memory on the card is dual-ported and zero latency so it can actually take these writes while simultaneously burning a good 400Mb/s-ish of reads to update the monitor.)
This resolution/bus speed mismatch is really painful on old ISA bus PCs; it wasn't unusual to have 50mhz 486 machines saddled with 1024x768 ISA video cards back in the day, and ISA is only really good for about 4MB/sec sustained. Even at an 8 bit color depth that's about 6 FPS, tops. GUI acceleration features could *help* but only so much. At 2.3MB/sec even a 320x240x8 bit video feed pushes the capacity of the bus; this is the reason why Macs and Amigas so badly dominated PCs in the multimedia department until the advent of "Local Bus" and PCI.