Although you change the HAL in Windows to get the extra CPU to 'show up', I recommend re-installing.
Couple of reasons -
1) The extra core if you re-install gives you more IRQ's.
2) The only way to take advantage of the extra IRQ's is a re-install as the IRQ tables are rebuilt.
3) More stability after a re-install using the correct HAL from the get-go.
E.g.
a) Originally I just changed the HAL from an APCI Uniprocessor PC to an ACPI Multiprocessor PC and got occassional problems.
b) I changed the HAL to an MPS one (not recommended by MSoft) and got no problems. Main difference is that MPS doesn't use ACPI therefore the IRQ tables rebuilt using the extra IRQs (128 in total).
c) Switching back to an APCI HAL results in occassional problems again, all related to IRQ sharing.
I'm re-installing on Sunday to finally sort everything out, the choice is yours.
Using the MPS HAL with rebuilt IRQ tables I gained 30fps. I expect similar after a reinstall using an APCI Uniprocessor HAL that can use the extra IRQs.