First off, a capacitor will not "bring the watts up even more." In fact, more than likely it won't do a whole lot of anything. Personally, I'd sell the cap and put the money towards upgrading your factory wiring (alternator to battery, and grounds) as well as an Optima battery. Second, I'd get the biggest possible amp...go for ATLEAST the 750w choice. Third, sealed OR ported...you're going to need lots-o air space. I'd go with ATLEAST a 1.5 cubic sealed box. Goodluck with it all, and be sure to post up some pics when you're done.