Quote: "You'll quickly get out of memory, each house interior and characters will add more to ram, if you have like Skyrim thousand houses on your level."
Well, I don't imagine he will want 1000 houses. Its the best way to keep people playing. Sure, you could load a level up for each house, but expect people to stop playing rather quickly. Game Guru loading times are a killer. You would only want a limited amount of interiors per level really. I don't think there would be a major issue. I haven't run out of memory in GG for a long time (apart from after several hours of loading and reloading).
If you needed a huge amount of interiors then you would be far better building your interiors out of sections, of which you would assemble on the fly when you enter the house. That way, you use the same segments, incur no real memory penalty and literally have as many as you want. Skyrim reuses graphics all the time also. A lot more work involved, especially if you have quests and interactive objects in the house. That way you could even assemble the interior at the actual location you are at so you can look out the windows and see your local around you.
There's all sorts of tricks to use really. Just depends how good you are in lua and how much time you want to put in really.
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.