Start with the system guide here, replace the speakers with Axiom Epic Midi system. However, do not get the sub.
Instead get the Yamaha YST-SW315, this is a true sub and can be had for less than $250 (you'll have to really search and check eBay). I have two of these, and when cranked, they make my neighbors dishes dance around her table, and she's 1mi - no two, naw threeeeee miiiiiiles away (actually ajoined townhouses).
If you anything left upgrade the DVD to a universal player like the Pioneer Elite 45A ( I do not know if this is an upgrade or just more money). Learn how to solder and make you own cables or get Sound King 12 gauge for the speakers, and AR (from BB) or Dayton (from parts Express) for component cables.