It depends on your router it also must have these features...mostly it is the number of antenna. It sound go in theory that you can keep transmitting more and more overlapping signals but each one you add add less overall because of the added interference. They just recently figure out how to get 4 feeds working and it does not work all that good even after many years of trying.
Generally 2 antenna (which is the 300 rate) tends to be the optimum price/performance point.
Unless the cost saving is really important I would stay away from brands like rosewill or belkin and lots of the other lessor known brands. Many tplink cards are close to the same price and you get much better support from tplink. Other brands that tend to...