Hi newcomer and welcome to the Tom's hardware forum.
Are the 500 Euros only for CPU? or for all the system. I suggest to you the Athlon II X4 620, is a very good processor, and quad-core for only $99.99.
Or, you could cut out a core and get this
, while it loses a core, it makes up for it in the greater speed of the other cores.
The CPU charts show that the Tri core gets 132FPS in L4D, while the Quad core get only 109FPS.