The best way to look at it is this: Normally a processor (1 core), can only do 1 operation at a time. The way the processor looks like it is doing as many things at once is because it cycles everything that is running at once through. (essentially, works on program A, then on B, then on A again, and so forth). Dual and Quad core processors are clocked slower, so each core is processing less commands per second say, but the multiple cores enable it to multitask. People like to compare it to having two service windows at a bank or something open instead of one. Where it gets a little complicated is that unless a particular program supports multi-threading, it can only use 1 core itself. This is often fine since you tend to have more than 1 process going on at any given time. Basically if you are running A, B, C, and D at 2.4ghz on a quad, it will take as long for the whole thing as it would take for one (a little less but that's the basic idea) where on a dual core running at 2.8 it would take less time to run any two of those, but more time to complete all 4 if they were running at once, and moreso if you were running a single core at 3.5gz say (not that its even close to worth getting one these days). Quad cores are almost always best if you are editing a lot of things, video, what have you, and a movie or game is less likely to studder if your virus scanner suddenly kicks up in the background (since it will use a different processor core than the game or movie is on).
The thing is, most games don't support multi-threading. Only newer ones do and then not all. If they don't, then a higher clockspeed may be important. The ultimate example people compare to though is Crysis, which does support it, and if I remember correctly, threads the AI separately from the environment and physics, taking advantage of at least 2 cores. All in all, anything 2.4 or better for a clockspeed is decent for most applications, and if you are doing anything that needs more than that you probably plan on overclocking anyway. So ultimately, a if it is the only thing using any significant amount of processing power, most games do better with less-cores and a higher clockspeed than getting more cores and accepting the lower clockspeed that comes with it. It is getting to the point though that the minimum clockspeeds on quad cores are within spec for games too old to support multi-threading.
If any of this is a little jumbled I would be glad to clarify.