Seeing as an x4 chip is slightly above the x6 chip I'd say its a wash. It's clocked 400MHz faster, but if it really needed all 6 cores then the x4 would have been slower.
Think of it this way if it helps. Even if a game needs two cores to work, more cores can/will help as the windows processes can use them instead of the cores that are playing the game. I'd say 4 cores are needed minimum. 8 cores might be needed in time, but we aren't there yet. (Yes the next consoles will have 8core CPUs, but they are bobcat cores which are very small.)
Best way is to look at the pic I posted. You have one big group at the top from the 3470 and up. You then have a middle grouping from the A10-5800K or so to the FX-4170. After that you have all the lower end stuff from the A10-5500 on down. Because they are in groups you won't be able to tell the difference between the PhII x4 980 and the i7-920. Or tell the difference between the 3470 and the 8350. The results they give are just to similar. Notice that in the examples I gave some of them are four core chips while others aren't. This is because you CAN'T judge performance just looking at core count. I really hope people learn this as I'm getting tired of people saying you need an 8 core chips once the new consoles are out. We might move that way, but its a ways out.
Edit: Seeing as you only care about CS:S what CPUs are you considering? Source came out when the P4 was around. You shouldn't have problems playing it on anything that came out probably 5 years or more ago. You are over thinking this.