far as I know 2 ports on a hub should work without power, if you want to use more you will have to plug in the hub (they usually come with a power adaptor). USB devices use 5 watts each.
another question for y'all...
my laptop which will be the "brain" of the carputer has a USB port (v1.1)available. i am planning on getting a USB hard drive and some other USB components. if i get a USB hub with say, 4 USB ports, will i need additional power to power these devices or will USB pull power from the laptop?
how can i figure out how much power is needed to operate all the USB devices?
Compliant USB ports supply upto 500mA @ 5v (2.5W) each. If you get an unpowered hub (i.e. one without an external power supply) then you can use upto 500mA (combined) for the devices you plug in. Don't forget, the hub itself uses a few milliamps too. You'll have to look at the power demand of your devices to find out if this will work for you or not.
If your going to use a USB hard drive (assuming you mean a 2.5" HD), that device alone will likely require the full 2.5W of power. In that case, you're going to need the external PSU for your hub. If you mean a 3.5" USB disk, then that will require an external PSU of its own and then you'd be able to use a unpowered hub.
Also, you should be aware that usb 1.1 isn't particularly fast and wont provide great data transfer rates to/from your HD. It's fine for GPS, mice, keyboards, and some sound cards.