Newbie in need of some help please....
 

Just bought my K2 Attack 2.0 about 2 weeks ago. got it for a steel because the crank needed replacing. I replaced the crank with a Truvativ 175mm crank with a 48/38/28 cog set up. After replacing this i can to find my derailleur wasn't up the the challenge of using all three gears (the new middle gear is 2 teeth smaller then the old largest gear). So i'm in the market for a new front derailleur but i don't know what im looking at or where to start. Some research i did is telling me a Shimano 105 but apparently it makes a differnce what your cog setup in the back is and i can only find setups for 9 and 10 speed in the rear and triple fronts when i need a 8 speed rear triple front. does this rear cog suggestion make a difference?

105 is a road der. shimano raod and mountain der. are not interchangeable. Each pulls a different amount of cable.

ok then what derailleur would somebody suggest

Are you sure the current one won't work with some adjustment? What does "not up to the task" really mean? If it won't clear the 48, just move it up until it does and then begin adjustment.