A 737-300 carries something like 30,000 pounds of fuel when fully gassed up. That "several hundred pounds" would be like the half-gallon of gas probably left in your car's tank when the needle passes "E" and the engine quits. I don't think they can suck every drop of fuel out of all three tanks, but I am just guessing.
There's other flammable stuff onboard--oil, hydraulic fluid, some of the plane's fixtures--but I guess they would take a kerosene fire to light off.
}:-)4
Several hundred pounds of fuel would be approximately 2100 gallons (based on the assumption of 300 pounds)
If you calculate that water is 7 pounds per gallon and kerosene is heavier, then that is only an estimate.
2100 gallons is sure a lot of fuel to consider a planes tanks empty.