Depending on needs, for a simple cheap fix one might consider the following:
In a Mac Pro 3,1 (Early 2008) running Mavericks 10.9.4, I replaced my failing (for the second time) ATI Radeon HD 2600 XT 256 MB with an EVGA NVIDIA GeForce GT 610 1024 MB, a low-cost (about $50) PC graphics card. This was installed in slot 2 based on reports that this slot is more reliable in this machine. Like the original, the new card has two DVI connectors (as well as one Mini-HDMI) and needs no internal power cable.
There are no graphics during boot, but it works fine for basic video with two 1080p monitors. Unlike the old card, it also provides basic CUDA and OpenCL support.