With Dell, you will NOT be able to Overclock, they use horrible motherboards that they lock the BIOS and even worse power supplies.
Also, there is not much of an upgrade going from a Q6600 to a Q9550, if it is sheer ghz you're looking for, get a good LGA 775 motherboard and new mid-tower case and an aftermarket heatsink/fan and OC the Q6600 to 3.0 or above, it'll be cheaper to do that than buy the @9550 itself...