Rendering animation and the time it takes

Rendering an animation at a high resolution can take a very long time. I ran some tests. I had a time scale of 15 seconds. 5 Spitfires flying across the sky, with an added cloud on the background. The estimated time of completion two weeks. After messing around by hiding some objects I found out that the cloud was the main cause of the speed, then the planes. There are many ways I can improve the speed of rendering. I can improve my computer hardware, I have purchased two sticks of 2GB RAM. Which will give me a total of 6GB of RAM, which should speed up my PC by a lot! Another thing I am going to have to do is reduce the resolution, instead of 1920x1080 too 1280x720. However the best thing I can do is reduce the polys and texture sizes of the actual models.I'm going to reduce the texture map to 512x512, x3 the size it was. On the plane I'm going to use the weld tool to reduce the polys, which will reduces them. The only reason I can reduces the detail is because in this scene the objects are far away and you will not be able to notice the detail that has been reduced.