Should be pretty quick! we build appliances with the Armada 370 CPUs (Single core Cortex A8, but same Ethernet and bus IP), and they can route/NAT at 800-1000 megabits between 2 interfaces (at 1500 byte packets)
PC has been a "router" for a long time.
Electric power costs for an old PC are significant. Don't bet against this being cheaper to operate than dragging that old Pentium box out of the closet. Depending on the size of your home, size and noise can be issues, too.
From now until March here in Scandinavia and a lot of central and eastern Europe the heat generated will simply add to the general background heat in the house and reduce the amount that has to be produced by the electric radiators. Of course if you live somewhere where air-conditioning is a necessity your mileage will vary :-)