See "Overclocking Dell BTX Computers" in tutorials. That computer has a 95W CPU limit. It won't boot with any CPU over that.
X6800 dual Core2Extreme. Should go 3.72GHz with Throttlestop method. 3.45 easy.
Q6700 Pinmod to 1333 FSB for 3.33GHz. requires pinmod for voltage also. Would be very stable. 1.5V. should do it.
Q9650 no overclock but fast chip as is.
X3xxx Xeons. Go to delidded.com for this. there are Xeons faster then Q9650 and under 95W.