What is the Dell part number for your laptop, please? Have you tested your laptop with the Win' 8 Compatibility Tester?
How are you wanting to install Windows 8? Do you want it to dual boot with Windows 7, or do you want Windows 8 to replace Windows 7 completely?
If you want to dual boot then you will need to resize the Win' 7 partition and create enough space for Win' 8 to install on and function.
If you want Win' 8 to replace Win' 7, then you will need to delete the Partition that Windows 7 is installed on. (Try to keep the Win' 7 restore partition if you can) You can delete that partition during the Windows 8 install.