Maybe it just isn’t possible in blender, but I tried to render a city by night time, where the light posts and a sun lamp lit up the scene. I used spot lamps and I’ve tried emission planes, area lamps and point lamps too, but no matter what I do the image gets VERY noisy… Only when I use 1000-2000 samples the image looks okay. Are there any solution on this?? 
You are probably right - I have to use another rendering engine… But thank you for helping me! 
I suggest that you think of it this way: those street-lamps are what are called practical lights. These are “things which (in the real world, at least …) ‘emit light’, and that appear on-camera.”
Pragmatically, those street-lamps don’t (or at least, do not have to …) “actually light-up the world.” At least, they don’t have to. The sun-lamp, entirely invisible to the camera, actually provides all of the illumination that is needed, except to the immediate surroundings of each street-lamp. But Cycles doesn’t distinguish between real vs. practical lights: if it’s in the scene and it’s designated as a light-source, then Cycles will dutifully compute for it. ZZZZZzzzzz…
So… I suggest… “Blender Internal (BI) to the rescue!” First, tell Cycles that those street-lamps really aren’t lights. They’re just objects, painted a nice, appropriate, “bright-looking” color. Render the scene to get your “base plate.”
Now, switch to Blender Internal, and create a scene … linked to exactly the same set of (just the …) street-lamps and also linked to anything they might need to “shine upon.” Here, the street-lamps are the (only(!)) light-sources: “world light” should be completely turned-off. Deal with any glow-effects here. What you should come up with is a plate that contains nothing but the lamps and their glow and their immediate effects upon the street-lamp posts, such that, when superimposed (“lighten” …) upon the base-plate, the final desired effect is achieved … through appropriate “tweaking” of mix-nodes that, by now, are dealing with two [MultiLayer OpenEXR] files containing the respective (Cycles, and BI …) render outputs. (So, this “tweaking” is “step #3 of 3,” and it is “computationally insignificant,” therefore real-time.)
It really, really bears repeating that … anytime and everytime you see a lovely shot in a movie, that “lovely shot” was not the first thing that came out of any render(er). CG renderers only produce “raw materials.” The goal is to plan ahead, to render once, to tweak and fine-tune at [comparative …] leisure.
Thanks! I am not very familiar with Blender Internal, but I’ve used it one time to render some thin smoke, which I composited on top of a cycles render. But I think that the only issue with this method is (and Im not sure of this) that it will be difficult to make the reflections from the light look realistic… - For instance… If I used a emision plane with a emission value of 1 in the Cycles scene to make the reflections, it would not make the reflections bright enough. But if I bumped it up to a value of, say 5 or 6, the 20+ emission planes would make the image noise… But is it possible to make the reflections look real with Blender Internal??
Anyway… Thanks you for helping! 