That one you built seems kinda power hungry. I cannot really imagine that being all that portable considering that it will chew through a battery very fast, even a 9 cell.
Granted if you don't use the video card and have it hindered that would help, but then why bother buying the one with the custom video card then?
I have an Asus x83 from Best Buy. Has a T6400, 4 gigs of RAM, 9300GS dedicated 256MB, N,G,B,A wireless network card, and 14.1" screen. I get around 3 1/2 to 4 hours on battery save mode.
Overall its a very nice laptop and will play most games (WoW, CoD4) on medium to high settings without any hiccups.
Oh yeah, and it is $649.99