The amps in PC speakers are an unknown, there are some that are good, but loads have a single chip that can put out about half a watt if you are lucky.
If you get the t-amp on a board, its proven to be quite ok. Plus its 12v so there is no probs putting it on the PC power supply, wheras PC speakers normally are mains powered so you have to make something to switch it.
And when you get PC speakers and dont use the speakers you are wasting money on them.
Just get something better then some one way paper coned crap like they wanted $200 for. A friend got some cheap 6.5" soundstreams for his car and they were pretty good for the price.