Upgrading my Mac Pro early 2009 - MacPro4,1 running OS 10.6.8, looking at GeForce GTX 750 Ti Graphics Card and wanted to make sure it would plug n play?
While your Mac Pro does have a PCI-e slot, it is only type 2.0, x16 width meaning that the GTX750 will not run at its maximum capability.
Additionally, recent Nvidia CUDA drivers for OS X do not support 10.6.8, meaning you would likely need to upgrade your OS to at least Mountain Lion (10.8) or greater.
So, no, it seems unlikely that your proposed card would work as "plug and play." This site may be of use: