I've been updating my wife's Mac Pro over the past few days since she doesn't feel she wants to upgrade to a newer one because of the ghastly cost. (She's a graphic designer, but her primary tools are InDesign and Illustrator followed by Photoshop.)
So far, I swapped the CPU tray with one from OWC (so lowest level 4-core Xeon to a 6-core mid-level Xeon -- still below chips now available, but represents a 50% boost in CPS power). I've cloned her 1TB HDD to a Crucial MX500 1TB SSD (but not yet installed since I forgot I had to get a drive adapter for her tray -- ordered today from B&H). So, the final upgrade of the triumvirate is the GPU -- replacing the antiquated Radeon HD5770.
Being a PC guy who is constantly tinkering with my computer over the years, I have multiple disused graphics cards. Of them, the two I thought of sliding into place are an Asus Radeon R9 280X and an MSI Radeon R9 390 8G. The latter is my preferred entry since it has 8GB of VRAM and represents a 50% increase in processing power.
So, my questions: What are the things I need to be aware of in this transplant? Macs, needless to say, are not as customizable and I can't just download the most recent drivers etc. Would OS X not already have drivers compatible with this since it runs AMD cards across the line? My wife runs two monitors. Do I have anything to worry about when running two monitors with the new card? (I have two 24" NEC monitor but have run dual monitors for years and so all my old cards handle this.)
Thank you for the help and guidance.