Should you overclock?
It really isn't a matter of should.
You can actually overclock for free. You just won't get as big of an overclock on a given set of hardware. To get a good overclock you need a decent heatsink/fan and you need good case airflow. One final thing is that your other components have to be reasonable decent or they will end up limiting your overclock.
Its kind of a bad time to upgrade because they are transitioning from DDR2 to DDR3. If you buy a DDR3 motherboard now you are future proof as long as it supports A 1333 FSB whick thay all do, but memory and boards are more expensive and the performance gain over DDR2 and accompanying motherboards is at best 3-4%. So is it worth is to pay twice as much for little to no gain in performance? In my opinion ..."no"
If you buy DDR2 motherboard, you will save a ton of cash cause the boards and ram are dirt cheap now. But in a year, maybe less, the price and performance of DDR3 motherboards and ram should go up and down respectively making them more desirable.
If you are like me and are always a year behind the times and fine with that then get yourself a P5B Deluxe or a DS3 (both are world class overclockers) or something. and some Half way decent DDR800. And the CPU of your choice. You will see no noticeable performance gain running DDR1200 over DDR800. If you benchmark both you will only see about a 1-2% increase in overall system performance and the extra cost just isnt worth it. You will not "feel" any difference between running $50 DDR677 and $150 DDR1000. I recommend DDR800 cause its dirt cheap and most all but the cheapest DDR800 will do over DDR900, the good DDR800 will do DDR1000 anyways. But it seems to be the sweet spot for Price performance.
Stay away from the E4300 and the DS3 combo. Although the DS3 is a good overclocker, with the E4XXX, it hits a FSB wall at about 370. Which is more than ANY E4300 needs to max out. But if your overclocking you want to at least have the options of lowering the CPU multi and raising the FSB up over 400 so that you can run your DDR800 at.. say for example, DDR900 and still have your memory ratio at 1:1. Which would be a 450fsb. Get a e6600 or something and with little OC your running is at 3.0Ghz. If you can get an old, early stepping C2D on Ebay or something you maight want to seriously consider it cause the early stepping C2D's could usually hit 3.6GHz on air no problem. The later runs of the E6XXX have problems hitting 3.0Ghz.
And.. just food for thought, if you already have a good power supply , case, and drives like I did and simply upgrade the core components. You can do the upgrade I have in my sig for $625 or so after rebates. And that is substituting the E4300 for a E6300.