Shouldn't slow you down, if you're purchasing a decent router. Most of the newer routers now will handle at least 20 megs throughput...many up to and over 100 megs now.
Some tips..ensure the router is running the latest firmware
Try manually setting the MTU on the router, 1500 for cable
Avoid using a router if the ISP supplied modem also has a router built in...you end up double NAT'ing. If you need to use your own router, disable the router in the ISP supplied gateway appliance..using the modem only in bridged mode. This is more common for DSL than for cable though.
Running any p2p programs? They tend to bog down most home grade routers with their high amount of concurrent connections.