Uhhh...
1) You will use 4 cores if you run any app that uses at least 2 cores (explained below).
2) Never say never in this business
3) Overclockability isn't the only thing that makes a good CPU, but it helps.
As long as you're running a multithreaded OS such as XP or Vista, anything else you're doing will be offloaded on to the free cores. I.E. if you're running a game that uses only two cores, those two cores can be dedicated to the game while the other two cores are dedicated to the OS. So even if the game or app you're running might not use more than two cores, you can still see a performance boost with a quad-core (however slight).
Intel CPUs have more than just overclocking going for them. They have efficiency, less power usage, more speed/watt, faster clock-for-clock, at STOCK SPEEDS than any AMD chip at this moment. While the overclocking potential for Intel chips are a lot higher, if you don't plan on overclocking, you will still have a nice lead on AMD in most circumstances.