First off what I would do is go to http://speedtest.net/
From there you will be able to test your download and upload speeds. Just so you don't waste any money what I would do is test your wifi speeds, then plug directly into a computer and test your non wifi speeds. If there is a big difference you will know that you are losing a lot of speed through your wireless setup. Sometimes something as simple as adjusting your routers settings or updating its firmware will do the trick, other times it's best to buy a new router.
Before throwing money at anything I would test out your speeds plugged in and through wifi just to be sure. Also, I'm not sure what package you're paying for through your ISP, but say for example you are getting 10 mbps, you may need to bump up your package if there are other devices using the internet at the same time. Like say someone is gaming while another person is streaming Netflix, it's gonna use up a lotta resources.