I note you said that you have read some forum topics, please check and read some of the recent replies as there are quite a few indications of what can work, and also include comments about order of importance in hardware (GPU first). Some however are in the licensed users section so unfortuenately you can't read those ones.
However, every situation, and budget, and scene etc is different. You determine what you are going to use Lumion for and work from there, and if things change then be adaptable, after-all its a business decision is it not.
Unfortunately users will not know what your $850 represents, in your dollars whatever they are that might be a lot to spend, or you are looking for 'budget' pc spending.
re: " i've friend who had build a new pc and lumion doesnt seems worked very well (Intel Core i7-3770K - GeForce GTX 660 - 16Gb .."
I have a GTX660 and it works really well, so it does depend on what you/your friend means exactly.
1. get the fastest, most powerful PC you can afford, and preferably a bit better than that, its an investment in income returns
2. the fastest, most powerful GTX or Radeon card (GTX tends to do better) so:
- Titan, then 780, then 770 or 680 etc....
- the more VRAM the better, min 1.5 but 3GB for the 780 is good, 6GB of the Titan is really nice. It would be nice if the 780 had 4GB but we're still waiting, it would be nice if all of them had more VRAM as that can help in some scenes with quality via higher resolution textures etc.
3. a motherboard and CPU that support the latest bridge - Haswell or Ivy
4. the fastest CPU supported by the motherboard along with the fastest RAM available for budget
5. 16GB RAM
6. quality parts do make a difference.
7. once you have bought or built it, there will be something faster and better tomorrow for Lumion that may do more, live with it
So the key question is, which product of Lumion will you buy.