I am personally running a Q6600 RIGHT NOW, and running all the latest shizznoodle and the
Q6600 is SOLID AS A ROCK - if you read your reviews carefully the good old Q still outperforms MANY newer mid range processors, and only slightly overclocked even still outperforms many higher range CPU's too,
but if it does have one flaw I could warn about
it is MEMORY BANDWIDTH.
Sadly this flaw is not at all to do with the processor , but just its general generation - at the time DDR2 800mhz was all the rage.
Now although mathematically the old Q6600 still has a LOT of punch, its overall memory-data bus is really beginning to feel the heat - a classic example is Supreme Commander - Forged Alliance, a game that with AI running, is notoriously brutal on a motherboard in general with its extremely large & wide floating maths stack it can bring almost any PC to its knees and this shows my point greatly - under full load the game begins to lag very badly, and if you go to a process explorer you will see that all of your cores are only rocking around 45% busy... 65% free but the game runs like crap - this is because there is upwards of 2.5 GB's of variables and co'ords and positions relating to every single unit on the map which is run in near full physics simulation.
The CPU cant physically GET at the work than needs to be done because the ram/bus itself is the bottleneck.
In reality, very little apart from massive physics simulation of literally thousands of units (Such as in SupCom FA) will show this flaw but it is there none the less.
You will also feel the burn on HIGH complexity HD graphics, and video trans coding - performance will drop but CPU will be less than 80% busy, the good old Q6600 was SO good, it was faster than its own memory bus.
So in short If you have a good Video Card, the Q6600 will still be taking on general gaming solid as a rock for another 2 years at least, but if you want to get into SLI/Crossfire high end performance configurations, you may be better off moving up to a nice 2500K i5
LATE EDIT : I will agree with anyone who has said it already, the 8600GT HAS GOT TO GO - I already threw out my 8800GT almost 2 years ago now - that will be by far the largest problem for newer games. If your priority is gaming spend as hard as you can on your video card, don't worry too much about the cpu, any reasonable quad core will do for MOST games, as long as you have a STRONG video card.