Wait a minute here behdadsoft...who told you that every engine has a max poly limit?
Let me straighten you out right now. There is no magic number of polys that can be rendered in a scene before the system stops responding. Polygons use virtual memory, and the more polygons you have, the more memory (RAM) is used. But the more memory you have, the more polys you can use, see the problem?
This is why when you are making models whether they be objects in the scene, or the scene itself, you always try to get the best look with the fewest polys possible. If you want your player base to be the 1% of rich people in the world with high-end Alienware computers that cost them $10,000 each, then sure, make a 500,000 poly terrain. I'd like to see a video of you playing on a half-a-million poly terrain actually. If you can do it, if you have the RAM, I"ll take all this back.
(I know I sound cynical, but wtf? 500k polys? *facepalm*)