What are you using the computer for? In gaming going from 3 or 4GB to 8GB doesn't make a lot of difference in performance. THG review of 3GB, 6GB and 12GB for gaming.
If you're running Photoshop or VMware, etc going with more RAM starts to make more sense.
The first thing you want to do is find out more about what RAM you have installed right now. You want to match that as closely as possible.
Run the CPU-Z utility program and the Memory and SPD tabs will get you the information you need.
Ehhhhhhh.... missed the part about the Dell computer. Sorry - Dell has locked out any chance of overclocking that E6850.
Sorry, there are no real upgrade options for you. Your E6850 is just about the top dual core CPU you can have in that system. And adding a quad core Q6700 won't get you enough of a performance improvement to be worth the cost of upgrading.