Edited by xtremerunnerars - 9/22/11 at 9:00am
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If you need the ability to play more than one source at the same time then you need a mixer. This would allow you to play your playstation (turn the in game music off) and your iPod/PC at the same time. This way you can play your own sound tracks to the games you like. Also once you get the mixer, you can start to play around with all the super neat DJ software out there. Traktor is my personal favorite.
Look at the behringer xenyx range for a mixer, but I wouldn't add unnessary compenents in the signal change and just change around the rca cables, you might need this to connect up your ps3 http://www.amazon.com/FiiO-Digital-Analog-Audio-Converter/dp/B0053VKP8S/ref=sr_1_1?ie=UTF8&qid=1316708265&sr=8-1 , cause the ps3 only has opitcal out , you will also need a line out dock for your ipod, here for a cheap sub: http://www.amazon.com/Dayton-Audio-SUB-80-Powered-Subwoofer/dp/B000I7VQAS/ref=sr_1_2?ie=UTF8&qid=1316708309&sr=8-2 .
As far as subs goes, $100 is a pretty small budgets.
This one at parts express fits the price range and has good reviews: Dayton Audio SUB-80
But you get what you pay for. One way to save some money is to shop on craigslist. It takes a while to find good deals, but I think you can get much more for you budget that way. Just don't buy anything if they don't let you hear it first.
Dang you JRG1990; you beat me to the sub rec.. ;)