I have a Dell Studio 540s Slim desktop PC with a Q8200 2.33ghz CPU & was wondering if I could UPGRADE the CPU without changing the motherboard or power supply. My power supply is 300w & I have seen in other forums that a Q9650 3.00ghz is the best CPU that will work with my current motherboard & power supply. I called Dell but they told me that they don't recommend using any CPU other than the Q8200 because it may damage my computer. Can anyone give me some advise as to whether the Q9650, Q9550 or Q9400 will work fine in my PC without any issues? Thanks! If dell recommended NOT to replace the original CPU according to OEM specifications and compatibility, then it is doing this at your own risk if disregarding their suggestion.Considering the relatively small performance boost you'd see from upgrading your CPU, I would probably advise against bothering, especially as the PRICES those CPUs often command on the used MARKET is a lot higher than I would SAY they are worth.  As that machine's a good few years OLD now and that CPU socket has been defunct for quite some time, you'd also be investing money into a dead-end machine at the end of the day.
If you're looking to boost performance, I would instead recommend getting an SSD - that would make much more of a noticeable difference than upgrading your CPU, and you'll also be able to move it to any semi-modern system as and when you get rid of that Dell so you wouldn't be wasting money in that sense.