The HAL only matters if you're swapping ISAs. By default the Windows x86 HAL will allow up to two sockets, and it doesn't really care about the number of processors.
Windows still needs to be syspreped before a chipset swap is done.
While generically true, it really depend on the combination of BIOS and HAL installed.
For example, with some older BIOS, WinXP install a "Uniprocessor ACPI HAL" or, even worse, a "Standard PC" one. In this case, updating to two cores will require a switch to an "Multiprocessor ACPI HAL", but this generally don't require user interaction as WinXP automatically switches to the right HAL (note: turning back to a uniprocessor system leave the multiprocessor HAL in place, so the system continue to run with a slightly lower optimized HAL).
On Windows Vista x64 and later, the system generically install itself with an "ACPI x64 HAL", ready for multicore/multiprocessor. So, in the poster's case, I think that he don't need to to anything to enjoy its newer dual core processor.