My 2c worth - I agree with chscag, you won't notice a lot of difference between FW (400) and USB2 (480), but Firewire is more consistent in its speed, whereas USB2 is 480 variable (depending on a number of factors).
And yes, you can go more cheaply with USB2, especially if you use a 'bare' hard drive (no enclosure) and a USB2 adaptor kit.