SLIZIAK,

You can click the link below to order memory from Dell. No, it will not void your warranty.

Dell Spare Parts

You can also call 1-877-717-3355

I tried but It always telling me "model not found" when I enter the service tag

The only way to know for sure is to remove the modules and look at the label - Dell buys RAM from everyone from Elpida to Samsung to Hynix to Nanya -- the only way to know is look inside.

Thanks for the answer, one more question. Where can I find any specification for RAM that is in that machine? Because I want the dual channel to work and dont want to buy 2x8GB because I am short on money now. Thanks

There is no such thing as "official Dell RAM'.  Dell buys memory from suppliers the same way you would - except instead of two modules at a time. they buy hundreds of thousands.

Crucial is the RAM to go for - they guarantee compatibility and no, upgrading memory will not void the warranty.

www.crucial.com/.../inspiron-15-(7559)