The thing is that you'll probably have to open the case sooner or later, so do it sooner.
Those ready sold machines usually have only 2 slots for RAM and it's pretty common for Dell to use them both like 2x256MB or 2x512MB. So you'd have to ditch one or both sticks to insert new ones.
The most vital information you (or us) need to know is the motherboard model. It gives all the necessary info what parts you can have. For that Everest is nice. Just copy paste the info here.
And while the case is open you might want to check the lable of your PSU and report the info also.
