Sonu_22 :
i know that the Xeon x5660 can be oc to 4.0 ghz but not to sure that can dell precesion t3500 can do so
If you are so pressed for overclocking .......... Instead of X5660 buy an used W3690 or W3680 on ebay or Aliexpress. X56XX series have locked multipliers but W3690/W3680 have unlocked multipliers.
Download and use Intel's utility XTU (extreme tuning utility) to overclock it on Windows i.e., without going into BIOS (Dell BIOS wont let you OC).
You should be able to OC to 4.2 GHz (or slightly more). ..... or at least 4 GHz ....... depending upon your luck with silicon lottery.