|If you have he fastest speed available and you are getting that speed or something close, buying a faster modem wont make any difference.|
Most Modems sold in the the last five years or so are capable of 8Mbs. Unless ATT are providing something faster than 8Mbs you are not going to achieve much.
The Dlink does have modem built in so you can do away with you existing modem. The modem is 2/2+ which means that it is capable of speeds of up to 24Mbs, but unless your ISP is providing that kind of service it wont be of any use to you. However, it is not wireless. If you want wireless you would better buying a modem/router that has wireless built in.