Our family Dell is really hard to disassemble. When I tested my new build I tried but failed to get the heatsink off. On mine there's a green cap covering the heatsink, sort of like an air tunnel, that was easy to get off but the heatsink wouldn't budge, even after trying to use force.
I hope yours would go more easily but I'd say that unless you want to put another heatsink on it, don't worry about putting Arctic Silver on your processor. The stock cooling does its job and you can't overclock a Dell (unless you download a pirate BIOS).