I've heard that you need to make modifications to get everything you are supposed to out of multi-core processors in windows XP. Does this stay true to SP3? Are all the necessary upgrades for multi-core included or does one still need to make modifications to fully utilize their multi-core cpu?
You are probably confusing it with something else. Windows will detect by default any extra processors and put them to work.
To actually see the number of CPU's detected you can go to:
In the window that says System variables, there is a line that says: NUMBER_OF_PROCESSORS
Next to that is the number of processors Windows are utilising.
If you have 2 or more CPU's but it only says 1, then you can click on EDIT and change it to 2 or whatever you have. But Windows by default uses the maximum number, so it SHOULD (in theory) be alright.
Hope that helps.