I do not usually upgrade laptops (aside from ram) so i will run through the basics that i know about computers in general.
NOTE: every performance upgrade you add/replace in your computer has the potential to drain more of your battery over time than it previously did.
To upgrade your computer from core 2 duo to quad core would simply be a matter of purchasing a quad core processor. To upgrade to an i7 (or any "i" processor to my knowledge) you would be required to upgrade the motherboard too. The processor chip the i7's use are different from the quad core and core 2 duo's. Additionally, the new motherboards would also require DDR3 (DDR2 will not fit).
As for the memory, solid state is the newest and fastest they make for internal drives currently. BUT they come at an expense, and honestly, you would probably find a 1.5-2 TB hard drive to be sufficient for what your looking at doing for the same price.
Graphics cards are a bit tougher. You will need to find out what graphics cards your laptop can handle. EVEN if it fits, your laptop might not be able to power a high performance graphics card. It all depends on what the laptop and battery can handle.
In the end, you would probably be looking at upgrading enough of your laptop to make getting a completely new one not such a bad idea.