Upgrading to Q9550 would definitely provide a performance boost; actually, the Q9550 is better than my current budget CPU, the G840 on socket 1155. Upgrading to Q9550 would be grand, however I doubt you would notice any difference from your current CPU on daily tasks. Your SSD is your best upgrade.
It depends on what you use this Dell machine for. If it's for gaming, a Q9550 and a decent GPU should allow you to run majority of the games out there today. As this is a Dell machine, packed with a Dell OEM motherboard; as you have stated you will not be able to overclock, or "tinker" with many settings.
If you're still using this computer daily, you could upgrade it if you desire. I also have a passion for upgrading old hardware to the best available/maximum upgrades; I always put the max. amount of RAM and best CPU in my old hardware.
Anyhow all the best.