You shouldn't NEED to place a Powered Rail every eight rails, unless you are trying to go up/down an incline. The best method is to use ONE Powered Rail, every TWENTY standard rails. This will allow you to keep momentum with you IN the Minecart. But, this only works on flat stretches of land. If you are working on an incline, you will need to place Powered Rails more frequently.
thats true, some people don't like duping. they think it's cheating, which technically it is lol. but assuming you want to grind...
it's about 500 meters (enough for the achievement) to get from the middle of the map to the other side, i believe. assuming the above post of one powered rail per 20 tracks, that's about 5 per hundred meters, and 25 for the whole way. i believe 4 powered rails is equivalent to 6 gold ingots, so thats atleast 42 gold ingots for the powered rails alone. and then there's the 178 iron ingots required.
have fun. ^_^
PS3 Username: WafflesUniversal hit me up some time.
as long as your on flat land, it will work every 30 blocks. so place your power rail, lay 30 regular rails and then another powered rail. least thats what we did the first week and there have been no updates so it should still work. that should put u right at 20 power rails