According to Dell specs and Crucial 8GB is the maximum. You may try 16GB but with no guarantee it will work. And you can upgrade the CPU but on a system the age of the 5720 money may be better spent on a newer faster system. This forum post has some information on a CPU upgrade.
Can I upgrade the Ram of Inspiron 17R 5720 into more than 8GB? Can I also change the cpu of Inspiron 17R 5720?
Upgrading RAM and CPU