Why are doing ethernet over USB?
And "faster" doing what? Surfing the web? Or transferring tons of files?
You could have a 100,000 MBps connection but it still wont make your 3 MBps DSL any faster.
You only need Cat 5e for 1000/gigabit ethernet connections, regardless, all Intel and G5 iMacs are 1000bt. So your iMac is capable, but you will only see the speed when transferring files across a network.