Here are the builds for that system.
http://www.userbenchmark.com/System/Dell-Studio-540/96
Q9650 is fairly common. I did see one QX6850 which is interesting because that's a 130W CPU. 9x333 65nm unlocked quad. This implies that a QX9650 should work, but I don't see any there.
When you talk overclocking and Dell there may be some trial and error involved. If you can get an unlocked CPU to run then Throttlestop software can get control of Voltage, and multiplier. The QX9650 is much better than the QX6850. But unproven on that system so far.
https://www.techpowerup.com/forums/threads/throttlestop-overclocking-desktop-pcs.235975/
Other methods of overclocking don't produce much unless you get control of Voltage.