Do I need extra cooling fan and extra power supply

The Q9550 runs fine with the stock fan if you're not going to overclock it.  If the power supply is not a well-known brand, I would replace it with a solid one from a reputable manufacturer such as PC Power & Cooling, Seasonic, or Antec Earthwatts.  The reason is a cheap power supply may fail at some point, and if it does, it could damage everything connected to it.

r.e. the advice to "... use a cooling fan ... other than the fan which is provided with the processor ..."   because "... it increases the performance ..."   ==>  An aftermarket heatsink has NO bearing on performance !!    As Callandor noted, the Intel stock fan is just fine -- it's designed to keep the CPU within its operating range, and does a fine job of doing so.    I generally DO replace it with a higher-end heatsink ... but not because more cooling is needed ==>  I do it purely because I like to build the quietest possible systems; and the Zalman heatsinks that I use are designed with very large fans which can run at lower rpms (and are thus quieter).    They are NOT, however, necessary -- and make NO difference in the functionality of the system.

As for "... Some dealers suggested me to use a power supply other than what they have already fitted in the ATX Cabinet  ..."  ==>  Since you didn't say what power supply is already in the cabinet, or what its specifications are, it's difficult to say whether or not you need to replace it.    I agree, however, with Callandor's comment that you should use a high-quality power supply ==> Seasonic and PC Power & Cooling are the only brands I use for my systems.    For the system you're building, any high-quality PSU of 450w or better would be fine.    Look at the actual supply in the ATX case, however, before you replace it -- it MAY be a reasonable supply.

if you are not planning to use gaming graphics, then the stock powersupply is okay.
if you are from india (i am from india too!) - then go for cabinets from color-bit or frontech.
if using cabinet from any other company, i would prefer if your powersupply was at least a Frontech or Atrix

which city are you planning to buy from?

in the indian summers an AC is essential to your PC's health. but if you dont keep your PC in the AC, then you must have at least 1, preferably 2 or 3 fans blowing hot air OUT from the cabinet.

what gary said, see what SMPS you are getting with the cabinet, then decide.

hathehariken.