With 5 nodes Twin X5660 Xeons = Intelligent configuration =)
RAW cost:
Twin X5660 = 1.300 euro x 5 = 6500 euro
(*) 980 GTX = 500 euro * 10 = 5000
(*) 3 PC nodes = 500 euro * 3 =1500
(*) TOT = 6500 euro
With 10x 980GTX you can do a lot of things, very fast for interior... but:
(*) 4 GB VRAM (forget complex outdoor projects)
(*) 10 GPU + 3 nodes eat A LOT, A LOT of electricity! More than 5 double Xeons, believe me!
Honestly, I prefer Xeon Config.