Assuming that your Internet "Speed" is high enough to accommodate so many devices and all of this devices are Wireless..
The Wireless capacity doesn't depend on the Number of devices per-se but on how many are used actively at the same time and what activity is they are used for.
If the Roku does not stream and all your devices are doing occasional surfing it will be lagging but it might work.
If the Ruko is on and few other devices are trying to stream Video you probably will have a lot of trouble.
Upgrade the Router might not be a solution since the E2000 is dual band Giga Router and higher level Router will not make in such a case the difference.
There is Not enough data to provide you with a definite answer but one way to get more from such Network is to install few more Wireless Routers configured as Access Points and connected via wire to the main router ( Using a Wireless Cable/DSL Router as a Switch with an Access Point - http://www.ezlan.net/router_AP.htm