I have dual monitors. A 2560 x 1600 main monitor which is used for games, and a second 1920 x 1200 monitor which has mostly static stuff like e-mail, web surfing, and performance monitors.
I recently upgraded my GTX260 to a 5870 with nice results. Windows-7 experience index for graphics dropped from 7.2 to 6.0. I was not dismayed by this because others have had the same experience.
I have long wondered if a second graphics card would be beneficial, even if not used for crossfire. Newegg had an offer of a 3450 for $15 after rebate, a deal! Installation was trivial, since it uses the same driver as the 5870. Running the windows experience index again raised my score from 6.0 to 7.8! I also found that the FPS of my usual game(civ-4) improved. Civ-4 is capped at 60fps, and previously, I would see it fluctuate between 55 and 60. Now it is pegged at 60.
All this makes sense. A video card with two displays must share the memory and other resources. Cutting out the need for managing an extra 1920 x 1200 monitor must leave more resources available for the gaming monitor. The 3450 is a relatively weak card, but it seems to be sufficient for non gaming uses.
Does anyone out there have any more input on this?
I think you already said it. You offloaded your second display onto a second card leaving the 5870 to run your 2560x1600 display. I used to run 2 displays off once card, and have it off to the side when gaming. But I set this up to turn off when in game and just use the primary display. I was noticing the same thing you were. Although this was MANY years ago, it was a similar experience.
Sounds like it worked out well, i will also keep this in mind myself. I have a little 15" lcd that I wouldn't mind using for a similar reason, but don't want to have it hanging of my 5870.