Truly a waste of money versus one of the core2duo (dual-core) options. If you look at real world performance in current PC games, this generation of quad-cores really isn't providing any boost in most of the games out there. Some game developers have said that in the long run, you may be better off buying a higher speed dual core instead, and the game performance benchmarks show. You pay the same price for this 2.4GHZ quad-core as you'd pay for the 3.0GHZ dual-core.
You may luck out as an overclocker and get one of thoe more overclocking-friendly chips (known as the g0 stepping or SLACR), but you won't always be guaranteed that. With those particular chips and some decent cooling, you can get your Q6600 to 3.0 and beyond depending on your cooling. However, it comes with risk, possible system stability pains, and the possibility of ruining your chip.
The choice is yours, but I strongly advise you to look at real world game benchmarks that compare current processors, particular for the games you intend to play, and see if your money is more wisely spent on a faster dual-core, because more than likely it will be. Some might say "oh buy you're investing in a longer-life product". Maybe so, but you really won't gain that much life out of it. After all, most people are upgrading every year or two nowadays anyways, and I doubt any person that interesting in buying such a top-end part like this will want to keep it much longer than when in a year from now you'll probably have chips that make this thing look like a Pentium 1.