I would recommend using two 2x6's sandwiched together with nails and a PL brand Construction Adhesive. Also apply the adhesive to the top of joist before you put it into place, this will help adhere your subfloor to the tops of your new joists (reducing squeeks, and overall bonding). I would also place them on 12" centers, you may need to shim your joist from the top of the foundation wall to get contact with your subfloor. This is not overkill if your concerned about the finished product, when walking on the floor you do not want any bounce. You will also want to put a support wall down the middle to support the span of your new joists. I would recommend a steel I beam. You can usually pick up previously used I beams from metal fabrication shops. Support the I beam with posts, that go down to a concrete foundation pad. This is just a recommendation, most contractors may call this "overkill", but if you want a finsihed product you don't have to worry about, do it right the first time. You didn't mention if you were going to replace the rim joist around the outside, if so, you could jack up your entire house and replace with all new "larger" floor system. You won't be able to get all the information you need from here. Contact a contractor and discuss your plans, and ask his advice. Most will give you advice at no charge. Make sure you get in contact with your city building codes, they may put a whole new set of rules out that you must go by.