Thanks for the suggestions. These will be for Optiplex 790 desktops, so I don't want to go with anything too powerful, and the 5770 starts getting up there. The 5750 is out of stock everywhere.
I ordered an eVGA GT 430 for testing. In the end I may just go with a display port -> DVI adapter, and use both of the onboard VGA and DP ports to drive 2 monitors. Not the most elegant solution, but for $7 for the adapter, it's worth giving it a shot. This would also utilize the HD 2000 driver I already injected into our image, and that makes things much easier to maintain as all workstations (single and dual display) will be running the same video driver.