Dell inspiron 1720 memory upgrade

I'm looking to upgrade my wife's laptop memory. It is a Dell Inspiron 1720, currently has 2gb of ram. I would like to put in as much as possible, of the best quality memory I can get. From the homework I've done so far I thought i'd get the below linked memory kit. I am not sure, however, about compatibility because I do not know anything about the motherboard inside. I've read online that there is a cap on total memory, with some debate as to what the cap is (I've read 4, 6 and even 8, but I thought I'd stick to 4gb to be safe).

Please tell me if this selection would be compatible with this laptop, and if it is not, what makes it incompatible? Can you suggest a serviceable alternative?

http://www.newegg.com/Product/Product.aspx?Item=N82E168...