hey !
I'd recommend a gamer card (a modern NVIDIA card, GTX 580 or newer ) simply because of the costs, thout you may run into issues when wanting to run 3 monitors. [ In general, we recommend NVIDIA ]
Though what you could do too ( and what I'm doing here ) to run 3 monitors is just to add a second graphics card in your machine. No problems here what so ever.
Btw. my opinion: If you're building a beastly machine anyway with 32 gigs of RAM, SSD and so on for working with CityEngine, I'd not try to save money with a too cheap graphics card. Don't take the PRO line where you pay $2000+, so I'd recommend something 'state of the art' such as a e.g. GTX 670. Why ? Because CityEngine models can easily go up to a million polygons ( or 10 🙂 ) if you want.
Need more inputs ?
Matt