Ok, so I don't know much about gearboxes, but is it possible to regear an auto box with a shorter 1st? Don't laugh, I'm a bit of a noob when it comes to these things... If not, is there any other cheaper way of increasing acceleration short of transplanting a 5spd?

try some zorst mods first..probably a hell of alot cheaper and if it doesent give the desired effect, hey theres one mod you can already cross of the list to do anyway

You generally wouldn't change just one gear as it would affect driveability too much, and instead you would change the final change gear so that all 4 gears would then be shorter, at the expensive of fuel economy.

You generally wouldn't change just one gear as it would affect driveability too much, and instead you would change the final change gear so that all 4 gears would then be shorter, at the expensive of fuel economy.
Yup you would have to change the gear ratios, so if you wanted a shorter first gear, your 4th will also be shorter which = not so good fuel economy.