Your current adapter is already getting the maximum speed you can from your ISP. If all you do with the PC is access the internet via the router then you will gain nothing from buying a faster nic card for your PC. It wouldn't matter if you put a 40g fiber connection between your PC and the router you will never get more bandwidth than you pay for....except in your case it appears you actually get 18m when you only pay for 15m so you have a very generous ISP.