Quad Core processors come in many different ways, now even with the Intel i7 series on the market, even from the same manufacturer. While AMD processors are usually praised for their budget price (and quad cores are no exceptions), and the newest Intel i7 for being too expensive and power hungry (rated at 130W!), the perfect middle ground is found in the Intel Quad Core Q9550 model.
Why do I say it's the perfect middle ground? For many reasons. Price is one of them. If you are looking for an AMD Quad Core Phenom processor because of its attractive price, I suggest to look a little further with a little extra cash. At least here, the Q9550 retails for less than 75$ price difference! And is it worth it? Oh my, in all conceivable ways, even compared to previous Intel processors (like the Q6xxx series or the Q9450 quad cores).
First, the cache from the previous model doubled from 6MB to an outstanding 12MB with just UNDER 20$ more (at least here). The frequency has also been slightly increased from 2.6Ghz to 2.8Ghz -- so if you were thinking that this processor lacks single-core power, try to come up with something else. It can easily be overclocked to 3.2Ghz with no conceivable problems or stability issues whatsoever.
It is also energy efficient too: it is rated at 90W, but that's at full load -- you don't have to use all four cores to have good performance, even one core rivals the best processors out there.
Intel have recently released 'more environmentally friendly' versions of all their quad cores which are more expensive but are rated at 60W, and in truth as some PC modders have found out, it's really not worth it. You can undervolt the Q9550 to a stable level to use less power overall, though I really wouldn't bother: the Q9550 is very efficient when idle and doesn't consume mindless power like some AMD processors (I'm not sure how the Phenom does, I heard it does better in this respect though).
Compared to previous Intel quad cores this one is a huge leap -- double cache means a lot if you know what you're talking about, far more than the difference between DDR2 and DDR3 RAM, especially if you access frequently used data (games, post-processing with multple effects on movies, etc).
So if you're looking for a powerful workstation-type of Quad Core CPU that is also affordable and not a power-hungry and heat generator like the newest i7s, do not hesitate to take a look at this CPU in action if you can. It is built on the latest 45nm process and it shows on its energy efficiency overall, especially when idle. So if you are afraid of wasting too much power or overheating your computer while just typing a simple document in Office, do not worry about it. Just make sure you supply it with a proper motherboard (that is, not a cheapstake) ;)