Thought I would add a little insight as I have tweaked my PC specifically for Zwift. It may or may not help.
Zwift PC requirements sets out three spec ranges; minimum (min) recommended (rec) & extreme (ext).
I was running i5 chip, (rec) 8gb ram (rec) and integrated HD4000 GPU (min). Ultra resolution was not available to me but high (1080p) was. On high I was getting around 10fps. Game experience was smooth but 3D imagery was thin (limited shadows etc) nevertheless still enjoyable.
I made one change by adding a graphics card. I chose a NVIDIA GTX960 (will come on to why later). This not only gave me Ultra resolution but I also get 60fps. The level of detail is awesome; shadows, sun rays and at night shadows cast by your cycling lights ( I tweeted Zwift some of the pics recently).
To be clear the level of pc spec wil not hamper your cycling efforts; this isn’t FPS lag elimination for competitive advantage. For the cost conscious you should decide how detailed you want your resolution to be. Aero position, knees screaming, ears full of blood and eyes stinging with sweat you may not even be looking for, or able to see, the shadows. Recommended specs are, after all, recommended. So in terms of building a spec I would say aim for recommended, if you can, and build any remaining headroom into the graphics card. If your graphics card is more than capable then less effort is needed elsewhere in the PC. Hence I added headroom by way of the graphic card.
So why NVIDIA?
Firstly I am no computer expert. Plenty of online PC sites will bewilder you with multiple statistics. NVIDIA attracted me because their newer line of cards utilise their Maxwell architecture that is particularly noted for 3D imagery coupled with their ability to run at much lower wattage compared to both their older cards and Raedon. I could have gone Raedon but I would have spent the saving and more upgrading my existing power supply. I am not recommending NVIDIA over Raedon. It’s important to remember you don’t need the best FPS 3D PC spec just one capable for your desired resolution. Zwift kindly provide a link to a video card benchmark site where you can compare your choice of card to their spec level suggestions.
Hope this helps.