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Prior to installing the processor, check that you have the latest BIOS version installed, when support for newer processors was added.
The procedures for replacing the processor are here,
http://support.dell.com/support/edocs/systems/inspd531/en/OM/parts.htm#wp1484335
As you intend to reuse the heatsink, plan on cleaning it's base and reapplying thermal compound to the processor.
Directions for cleaning and applying thermal compound, are HERE.
Only need a small tube of thermal compound is needed, this can be purchased from any online, or local computer store.
Sorry, cannot help with buying a SATA hard drive in the UK, as I reside in the US, but a good place to locate a hard drive at a good price, would be to browse eBay.
The Inspiron 531 default 300w PSU will support two 3.5" internal SATA hard drives.
Bev.
