Hey Puffy!
There are alot of factors there...
Cheap cards work well, Expensive cards work better...
Cheap switches work well, Expensive Switches work better...
It all really depends on your setup... Hanging 24 users off a $150 Dlink switch will run alot slower than hanging the same 24 users off a $600 HP ProCurve Switch...
Routers etc all work the same as well... Starting 10 BitTorrent downloads at once will pretty much bring a cheap linksys router to a crawl... But 100 BitTorrent downloads at once wont even move the activity lights on a Nortel Networks Backbone Link Node Router...
On board Ether uses the same components as Cheap Ether Cards, Unless you are buying a server board from intel etc...
I you have a few machines around the house and can afford it go with Gigabit Ether as it is wicked fast for moving files, movies, etc... around from machine to machine...
If you've just got the one computer, then pounding on a 5 Mbit cable modem with a 10Gbit ether card is pointless... an onboard ether or a cheap card will suffice...
IMHO for
MOST all home applications cheap ether gear works just great...
Hope this helps