With that configuration, you might be able to get through a very short DVD before you run out of juice. You really need dual battereies to watch a movie longer than 1h30min or so. On my dell p3 866-M, 15" SXGA+ TFT LCD, 8X DVD-ROM with dual batteries, a 2+ hour movie drops my batteries to around 20%. I haven't watched a 3 hour movie on it yet, but that would probably drop it down to the 10% range. Playing a DVD sucks down batteries faster than pretty much anything else.
Its a good thing you're not one of those guys who's decided an athlon based laptop is the only way they'll go, because if you used the athlon 4 in there, you probably woulden't be able to make it through a single short movie before your battery died.