I would recommend getting an external HD case that has a USB and an eSATA interface.
The cases are fairly cheap and can use any 3.5" SATA or SATA2 drive.
First, test the drive using the USB interface, if the performance is good enough for your needs (I doubt it won't be), you'll have saved a lot of money. If the drive isn't fast enough, you can always buy an eSATA card.
I doubt your needs will require the speed difference offered by the interface change. For that matter, I doubt most hard drives on the market are even capable of saturating the USB interface.
There's no point getting an eSATA setup for a drive that only has 100MB/second throughput.
The only way the USB interface wouldn't be fast enough is if you have many USB devices connected to the same bus. Because of how USB works (polling protocol with channels), extra devices can slow things down. If your laptop has other devices that use up a lot of USB bandwidth (a mouse and keyboard would not count), then I'd suggest going the eSATA route.
Firewire isn't very popular for external hard drives. I would avoid getting a Firewire setup.