You need to make sure your current WiFi router really needs a replacement before spending money on a new one. Use a WiFi analysis tool like NetSpot to quickly discover all surrounding wireless networks and see whether the issue might be caused by them crowding the same wireless channel as your router is using.
Also look around to see whether there are any physical obstacles near the router that might be just obstructing the signal from getting to where you need it. Some interference might be caused by nearby wireless networks and other electronic devices as well. If everything seems great otherwise, then it may be time to really upgrade your wireless equipment to achieve a better wireless coverage.