With a $2 cable (and possibly an adapter, for a total outlay of ~$5, including sales tax), pretty much any MP3 player can be hooked to any home audio system. Contrary to what some may believe, you don’t need to get something that has the Apple iPod docking connector to get your iPod to blast the walls down.
I do not consider any of the speaker docks to be loud enough for a decent party, especially not any of them that are reasonably priced. Those docks are generally designed to merely allow for leisurely listening without headphones/earbuds and are not designed to fill a space with sound, especially not a space full of people who are anything other than silent The Bose system that @MissAnthrope suggests cannot compete with the 1000W 5.1 surround home theater system (with iPod dock!) I got for less than half the price, and any lesser dock would be even more inadequate.
My suggestion is to just get the adapters required to hook into the aux inputs of your stereo and go from there. If you really are not up for moving the stereo then things may get tricky and expensive, and I have never been impressed by most rental PA systems anyways. Hell, it may be cheaper and give better sound to buy a home theater system like mine and install it in the basement permanently rather than rent anything!