Value of FNOSC in the configuration of my code
If you want to use your 8MHz crystal directly, you need to switch the PLL off. As it stands you have the x4 PLL set on. Your clock frequency will then be multiplied by 4, so 8MHz * 4 = 32MHz, which is what you are seeing.

If you check the datasheet, it shows this information in the the clock diagram:



So, read the oscillator section of the PIC24F Family Reference Manual carefully, and make sure your config bits are set for just the external HS oscillator and PLL off. The config bit options will be in the header file for that part (probably at the end)