Buy some minifigs on Bricklink.
I'd think, like, 10 or 15 each of some cool factions, then of course weapons, vehicles, and terrrain.
EDIT: Or, you know, a few Imperial Dropships
and some other cool sci-fi faction. Agents look nice, but you can't get a volume of them. Personally, I would buy a few dropships and Bricklink some Agents. Yeah. I would.
Stormtroopers vs. Agents. Shiny.