As Mattjd says - definately check the top of the CPU and see what it says.
As above the code "SLACR" means it is a G0 stepping - newer chip, lower wattage, good overclocker.
The other option is it says "SL9UM", which means it is a B3 stepping - an older chip, higher wattage, not so good overclocker.
As for quitness, there are several things you can do - all work towards eliminating or mitigating the effect of moving parts and vibrations within the PC. It is probably best to look at this on a component basis:
Case cooling - as suggested above the
Fractal Design R3 case is excellent for quietness. It has sound insulation in it and the HDD trays have rubber washers to prevent some of the vibration. However this case does not have particularly good cooling at stock (especially if you want to run some components passively) so I'd suggest adding at least 3 more fans (
sharkoon,
noctua or
fractal design models would be good) and a
fan controller so you can tune the fan speed to achieve the best balance of cooling and noise. The
Antec P183 is another good case if you want to keep things quiet.
CPU - The CPU cooler is a crucial part as it is a key component for any computer system - especially an overclocked one. If you merely want to run the system quietly, some thing like
this would be fine. But if you want it as close to inaudable as possible then you should have a look at the
Thermalright HR-02, this cooler is so good it can passively cool and overclocked i7 - so it will certianly cool and overclocked Q6600 (or Phenom II or i5 is you decide to buy something newer).
Storage - Sometimes a bad hard disk can drive you crazy with noise. Most modern 7200RPM hard disks are relatively quiet - especially once within a case. Something like
a Samsung F3 would be what I would pick. You could go for an even quieter 5400rpm drive like
a Samsung F2 Ecogreen - but imho the performance and responsiveness drop is not worth it if you are using it as a primary drive. Another option is to get a cheap SSD and use a cheap mechanical drive to run your other files (something like an F2). Something like
this may be worth a look - but with your budget it is a big expense.
PSU - Something like
this Antec 550W will generally be nice and quiet - but it does use a fan. If you can afford it - something like the
seasonic 460W fanless would be excellent - however it is mightily expensive considering your total budget.
Optical drive - not really an issue if you use non-DVD based games, however when it is on it will be pretty loud.
Graphics card - Probably the toughest obsticle to overcome. As you want to play games then you will want a powerful graphics card - however these cards use a lot of power and generate much heat. This means they are usually ill-suited to passive cooling and need fans to cool them down - introducing noise. You best bet here is to either buy a decent graphics card with a good cooler installed or install an aftermarket cooler yourself. If you want to go down the passive route - the most powerful card with a passive cooler appears to be the AMD HD 5770 - as Gigabyte have recently lunched a
passive model.
Hope that is of some help.