Paladin is correct. Many have a 95W CPU limit, some chipsets can support Xeons, others can't, and then there's the locked BIOS question.
What motherboard, or OEM system?
I'm going to guess since you say it overclocks it doesn't have a locked BIOS. Some boards that OC have the 95W limit.
95W LGA775 CPUs are Q9650,Q9550, Q9550S (65W) X3370,X3380
>95W LGA775 are QX9650, QX9770 (136W, 400 fsb)
If the chipset supports xeons-
LGA771 Xeons QX9775 (unlocked dual CPU 150W 400FSB)
X5450,X5470,X5460 120W
E5450 80W