CPU decision

Hi. So, i'll be buying myself a new rig, but since i'm living in Croatia it's a biiiit more complicated, the prices are omfg. So this:

GPU: Sapphire Radeon HD 7850 OC
Mobo: ?
CPU: ?
RAM: Corsair Vengeance (2x 4gb) 1600 MHz CL9
PSU: Corsair GS 600W

FIRST: I need a CPU mostly for gaming, actually pretty much only for gaming, and games i will play would be, well, everything like The witcher, Mass effect, Darksiders, maybe even Sniper:Ghost warrior, but most important is Guild Wars 2 i'm thinking to play, which looks like it will be optimized for quad cores, now there's my problem.

Now there's a prob. I can't decide the CPU, which then would decide the Mobo.

So i was thinking, since i5, one of the better, 3550, in Croatia costs 340 $ no more no less, no way i'll go for it.

Then i was looking on i3s, and i don't know, i thought of buying i3 2120, which would be just my budget,180 $, which is in the same time a really good CPU, how much i know, but it's only a dual core.

Now there's AMD's. They have low budget, my budget, quad processors, so i thought of FX 6100/4100, or even AMD Phenom II X4 965 Black Edition.

I have read a ton of reviews, and i just can't decide properly, is it worth buying an i3 even if a game i will be mainly playing isn't optimized for dual cores, would it be a lot worse or a bit worse?, or should i buy AMD quad, which are, how much i know, just a bit worse than intel's i3... or should i pay like 160 $ for an i5 ?