Check which RJ45 cable you have. If you have CAT6 or even CAT 5e, you are fine, both of these are capable of handling speeds upto 1000 Mbps, or a Gigabit per second.
The category should be written on the cable itself.
3090FE has 350W TDP. I am running mine on 850W PSU, even under full load it never crosses 650-660W. I am running everything on stock though, nothing overclocked.
The wattage breakdown of my rig comes around 640W: