In general Nvidia charges more for their stuff, and have certain quality requirements and testing that end up making there cards cost more. In general Nvidia cards are more consistent between manufacturers, AMD cards not so much.
--- What would be the gain for upgrading the Internal Ethernet Network Card from standard to:
Intel Pro 10/100/1000
Probably not much, most MotherBoards come with 10/100/1000 ports already and for 99% of people buying the motherboard that's good enough. I guess the biggest upside is having a 2nd port if you think 1 of them died. Otherwise unless you're doing something specific you probably won't need it.