There are three speeds you need to take into account with wireless routers:
Wireless LAN speed: This is the max speed you will get between the router and all computers connected to the wireless router cumulatively. The speed may be lower if there is interference from other sources, or even just from heavy traffic from your own wireless devices.
Wired LAN speed: This is the max speed you will get between the router and each computer connected via ethernet. Good wireless N routers will give you speeds up to 1 Gbps on the LAN.
Wired WAN (internet) speed: This is the max speed the router can connect to the internet/outside world with, shared cumulatively to all devices on your network whether wired or wireless. You want to make sure the router supports at least the speed your ISP is selling you, but anything more will not be used, as you're capped by the ISP. Rarely does an ISP provide faster internet than the router can connect at.
The LAN speed is important if you have multiple computers on your network and you transfer data between them. Let's say you have a home server connected via ethernet to your router and a laptop connected via WiFi, and you want to backup your laptop to the server over the network. The faster your WiFi speeds are, the faster that backup will run. Same goes for playing networked video games, transferring media files, streaming music, etc.
Devices on your network need to have the appropriate hardware to connect at these speeds as well. If your laptop's wireless card only goes up to 300 Mbps (which most decent-quality 802.11n cards do), then a 450 Mbps WLAN doesn't do you any more good than a 300 Mbps WLAN. Likewise, if you have a low-quality Wireless-N card or a Wireless-G card (which can only operate at 54 Mbps), then it doesn't matter what speed your router can connect at, because you will be limited by the slowest piece in your connection chain.
When deciding on what network equipment to use, take into consideration what your uses will be. If you do very little local area network transfers, then there is little or no advantage to having a Wireless-N router if your internet connection isn't faster than 54 Mbps. On the other hand, if you do transfer a lot of files locally, then a fast router may be a good idea, even if you have a very slow internet connection.