A couple things worth mentioning, since the impetus behind your wanting to max out your RAM is due to perceived slowness of the computer.
1) 4GB (minus a tad for the onboard video) is a respectable amount. Unless you are doing video editing or rendering, or are in the habit of having dozens of things running at once, it ought to serve for most other purposes. Without knowing what you use the computer for it's hard to say if adding loads more RAM would be a dramatic improvement.
2) Which brings us to another issue. Dell is fond of putting a lot of their utility programs on their computers and having them all load at startup. I'd recommend running Startup Inspector for Windows - Free and then
use the Snipping Tool
to post a screenshot of what it finds. Chances are that you have stuff running behind your back that you don't need.
3) Fill out your complete System Specs - someone with eyes sharper than mine may have some suggestions.
4) Your CPU is showing it's age a bit, it's listed as a Intel® Core™2 Duo Processor E6550 2.33 GHz. Don't expect miracles.
On the whole, if you're mainly using the computer for email and the Internet you can get by for a while. The fact that you're locked into (what I discovered yesterday) is still pricey DDR2 RAM means that I wouldn't sink a lot of money into upgrades. When the time comes, you'll get far more bang for your buck by saving the dough now and putting it into a brand new system later. I am one to use my computer until I run into something I really want to do and can't, due to the limitations of my current hardware. Unless someone is made of money it makes no sense to upgrade "just because".