You should have a DHCP table that you can look at on your router's page (as you have a Belkin, the default is: 192.168.2.1, not 192.168.1.1 as previously mentioned).
You should only see the same number of internet connected devices as what you have going in your house (not sure if you have a printer, network storage, Wii, ipod, etc).
When your connection slows, check your wireless connectivity.
If that is dropping, then you have interference from something.
Usual culprits in my experience are microwaves & cordless phones operating on the 2.4ghz range.