The biggest bang for the buck without a doubt is "SE" (3.89:1) differential gears. They run ~$80 new and you can swap 'em out in your driveway. Not a huge increase (+6.5%) in performance but not a huge price, either. Of course your highway mileage will go down a similar amount and your speedo will be off a little.JE.Electronics said:Nitrous.
$800 or so.