Be aware of older GPUs, especially NVIDIA. Current drivers required by newer operating systems sometimes won't work with older GPUs. Moving to the hardware forum, as well.
You really are going old school and budget!
Try to find a good used GTX 480. They are REALLY affordable now, and have excellent memory bandwidth. Mine worked really well back YEARs ago when I too was running CS6. They don't have a huge amount of RAM, but unless you are cutting 4k and you have a 4k monitor, I think that you'll be OK.
Thanks for the heads up, Kevin!
So, would I be better off with simply getting a more up to date GPU? My motherboard only carries a single PCI Express 2.0 16x slot, which originally made me conclude that the best GPU I could get was a GTX 580, I've since heard various reports that using a card designed for PCI-E 3.0 on a 2.0 slot would be fine, is this correct?
What performance reductions might I encounter with using a more up-to-date GPU on a PCI-E 2.0 slot?