If budget is the issue, then I would suggest going for AMD FX 8150 or similar CPU as despite disappointing online reviews, in our own benchmarks, it appears to be a very powerful CPU. You can go for some cheap DDR3 1333 Mhz RAM and later upgrade to something like DDR3 1600 Mhz or higher. The only noticeable difference in RAM speed you will see going from 1333 Mhz to 1600 Mhz (mostly due to improved latency), so it's not recommended to spend on anything beyond that.

If you want to go for Intel, then you can go for cheaper last year's Sand Bridge CPUs, Core i5 2500K would be a very strong CPU, or you can even go for Core i7 2600K, although combined with motherboard it will eat your 400€ rather quickly. The benefit of going for those is new Intel HD 3000 integrated video card, which finally supports multisampling, OpenGL 3 and is a pretty good deal. It also seems to work in Linux very well.