Boston Acoustics speakers aren't worth their price.
You are the final judge when it comes to speakers so I recommend you make your way to a local car audio store (not BestBuy/Circuit City, but an independent store) and listen to some speakers. Bring a mix CD of your favorite songs so you can demo them. It will be pretty obvious which ones are for you based on price and sound. Expect to pay between $150 and $275 for a pair of decent separates. $350-400 will get you a nice set and $800 will get you a really nice set. Brands like DLS, Focal, Diamond, Alpine, and Infinity make nice speakers but don't limit yourself by brand. The P5 has 6x8 speakers up front so you can fit 6x8, 5x7, 5.25, and 6.5 inch speakers. Round speakers require a little additional work but it will broaden your selection to choose from.
The setup I recommend to everyone is a set of separates/components, a small 8" or 10" subwoofer and a 4 channel amp to power them. Rear speakers can remain stock and the stock head unit can be used or upgraded at a later time. The following setup would cost anywhere from $500 to $2000 depending on the amount of bass you want and level of sound quality you want to achieve.
Check out the $500 and $1000 audio challenge sticky threads for some ideas on equipment.
