Hello Haitham
the throughput of 2821 seems to be 455 Mbps.. this URL looks a typo, with 3845 given 2 times... i think the throughputs are as given below:
Cisco 1841: 125 Mbps
Cisco 2801: 127 Mbps
Cisco 2811: 130 Mbps
Cisco 2821: 455 Mbps
Cisco 2851: 530 Mbps
Cisco 3825: 855 Mbps
Cisco 3845: 1.1 Gbps
Hope this helps.. all the best..
Raj