I curently have a "decent" system in my old BMW. I'm thinking of selling it and putting in what I already have into the S2000.
HU: Eclipse CD5000 (bought for $100)
Sub: Sony Xplode 12"
Amp: Sony Xplode 760W amp
I know the Xplode line isn't the best in the world, but I bought the sub (in a box) and amp from a buddy for $50. Not looking for hi-end sound system, just want to save some money and use what I already have. I just ordered the IR DCI from modifry last night. I found a trunkwell enclosure off e-bay for $105 including shipping. Do you think the a 12" sub is too much if I turn the amp power down? And do you guys (and girls) have any major problem with the trunk rattling too much? If so, how did you solve it?

 

I would strongly consider a second amp and some good front speakers. Then you will be "ok." The stock speakers are the weakest link in the chain, and if you don't address them now, you WILL be addressing them soon.

So, save yourself the hassel of taking it all apart now, and do the fronts.