If you're planning on getting the DLC then Operation Anchorage is an easy way to top up on the human kills. It might be a simulation but it's chock full of humans and they all count.
Update: Operation Anchorage is also available as part of the Fallout 3: Game of the Year edition.
First quest location details (credit to fallout.wikia.com):
Operation: Anchorage will be activated by a radio broadcast stating, "This is Defender Morrill, any Outcasts listening on this frequency report to sector 7-B, Bailey's Crossroads. This is a high-priority message, backup is needed at our location. Any personnel listening on this frequency please report at once." A marker will appear on the world map near the Red Racer Factory.
[+thanks to http://www.archive.org/
. This mini-edit was lost in the big TA crash of whenever it was but not lost to the WaybackMachine :-)]