PreI think others in here have probably already explained how accelerated graphics can help overcome the bandwidth limitations of the ISA bus, by speeding up the way various line draws are performed. You can think of it as bitmapped graphics (VGA) versus vector graphics (8514). Windows 3.1 is an environment where accelerated graphics makes a HUGE difference.
As others have said, having lots of memory helps tremendously with screen resolution and colour depth. Windows 3.1 doesn't really make good use of 256 colour graphics. Personally I find the windows dithering revolting. Having 15-bit, 16-bit and 24-bit colour depths makes the win31 experience so much better. Though you have to keep in mind that there are also quite a few win31 apps that make extensive use of 256 color cycling that won't run under other colour depths.
My all time favourite ISA card for windows is the Mach64 VRAM. The 2MB version is probably all you'll ever need. It offers 1024x768@16-bit colour. I have the 4MB version, which can do 1600x1200@16-bit colour, but it's really too slow to be practical. Probably your best bet is to pick up that CL GD5434 card I mentioned earlier. They're usually not too hard to find, and pretty cheap. Great DOS performance, as well as pretty darn good