how loud to you want to go? you could get a much better-sounding system if you upgrade the front speakers and upgrade the head unit (receiver). for speakers, i really dig the inifinity kappa speakers. they have a high sensitivity rating which means they'll be louder without needing an additional amplifier.
for head units, there are lots of good ones, but i prefer the kenwood KDC receivers.
find a head unit on sale (A GOOD ONE THOUGH) for around $110 and the rest you can use for speakers.
$250 is a tough budget to work with. you won't be able to upgrade all 4 speakers plus get a new receiver for that kind of cash