1. What is the advertised speed you're paying for?
2. Yes, get the fastest router you can afford. But...that is only a theoretical speed, especially wireless. Many factors can impact that speed.
A faster router will only talk to the outside world at whatever speed your ISP gives you. But inside your network, talking to another PC on the same router, you may begin to approach that 300Mbps.
If both devices are wired, you may begin to approach 1000Mbps between the two.