IMO it wouldn't be worth the difference, you'd have to gear it very aggresively to make a good difference and then your fuel consumption would be utterly crap on a run! plus you'd actually have to find the right box (could be easy could be impossible and buy that)so in conclusion, not worth the expense or hassle for me
If you want to keep what you've got and you want to change the gearbox, change the engine aswell

but to do things properly you'd want/need brakes and suspension sorting so it would turn out very expensive to pay someone to do it, IMO a new car should be on the cards...
