There's absolutely no reason to pass on a nice upgraded A2000. I have both an A2000 and A1200 setup next to each other, the A2000 easily gets the most use.
AGA is really a must if you like demos, and of course there were a few awesome AGA-only titles. Most AGA games offered very little over the OCS versions, though.
If you only want one system, want a gfx card, and must have AGA, get an A4000. Just make sure the battery is removed and all the electrolytic capacitors are replaced before putting it to serious use.
With a gfx card, you'll either need 2 monitors, or a card that will scandouble and pass-through the native output like a Picasso IV or CV64/3D (+ extra scandoubler module). A few graphics cards have a straight pass-through (no scandoubling), most useful for an A3000 which came with its own scandoubler.
Another option is to use a monitor switcher (yuck) or CRT with dual inputs (much better), and switch between your gfx card output, and an Indivision.