You';re link doesnt work .. so we can't
Did you confirm with Dell that your system would expand to 8gb?
If it does then check if it will take a 1x4gb stick; I dont think it will and that you are limited to 4x2gb if you want 8gb's of ram.
Here are your choices at Newegg:http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007611%20600006042%20600006071&IsNodeId=1&name=8GB%20%284%20x%202GB%29
Crucial dot com will also have some available at reasonable prices. They are Micron's brand name so that means they actually make the memory chips they use unlike the other vendors.