I have the chance to buy 4Gb of DDR3 ram for only 15 euros, but i was wondering if it would affect wow in any way? It won't probably increase my FPS, but what about Loading screens? I also have issues with UI lag ever since 5.0.
What OS are you using? If its not 64bit, chances are the additional 2gb of ram will do next to nothing. Most of the remaining address space is likely used up by the GPU.
Run a performance monitor along with WoW, see whats maxing out. Neither your CPU nor Video card are that impressive, but i'm guessing the GPU is a bigger FPS bottleneck. If you want loading screen speed/texture pop-in increased, consider an SSD. That is where you will get the biggest boost in that regard.