I have this Speedtouch 500 series modem, with an Ethernet port and a USB port. The questions like this. If I plug in the Ethernet to my computer's network card and also plug in the USB. Would I get better speeds or will it be the same as only plugging the Ethernet??
Similarly, if I have 2 network cards and both are plugged in to the same router, would it be faster??
No, it would probably be slower if it worked at all.
You would need to chose which interface to use as default - I recommend that you se one ethernet connection per computer for the best connection.
In any sort of a normal circumstance, the USB will be significantly slower (if it works at all) than the ethernet. I have never heard of anyone using both or even the usb for anything more than changing modem settings.
if that's a dialup modem, it wouldn't matter either way, as your connection is way below what ethernet or usb can handle...
Unless your modem is a faster than a T1 connection, ethernet should be more than adequate for your purposes.