As @plemans suggests, a lot depends on what you are trying to do and what you are using on your network.
In many cases, getting 2000 Mbps would be useless. Most current LAN stuff maxes out at 1000 Mbps. This is bound to change over the years, but busting a gut to get there now may be pointless for most people.
There is a 2.5gb pc connecting the connection!! I will download the file from the internet (movie-Game) There is a network switch with 2.5 gb support Will I do the defragmentation with these devices? CM2000-> RBKE963B-> Trendnet TEG-S380-> GS716T->
While more tangentially related. when using a dual WAN router with 2 different ISPs, you will not get 2Gbps on everything. For example, many services that need a connection to a single server, or a single TCP or UDP connection, or any services that uses some form of DRM to ensure only subscribers are connecting, will typically only use one of the connection. The router will simply load balance the connections.
The way this will look to the user, is you may see a steam game only download at 1Gbps even though you have 2Gbps worth of internet connections. but if a second user also starts a steam download, that may also end up going at 1Gbps. On the other hand, you may notice that some more P2P related stuff may benefit from both connections, in addition to other traffic that will make a large number of connections.
PS if you plan to run any servers, e.g., a game server, then you may be forced to restrict the load balancing on a client by client basis so that sessions remain more sticky when it comes to remaining on the same WAN interface.
