You options are pretty limited with a budget of only $250. My first move would be to replace the head unit with a good mid-range Sony or Pioneer. I believe you can even get Alpines for that kind of money now. But this can get to be an expensive hobby. Let me show you what I mean:
Head Unit:
Alpine IVA-C800 $2,130
Amps:
Rockford Fosgate 200a4 $ 350
Crossfire 100x2 $ 360
Speakers:
Boston Accoustics Pro Series 5 1/4" Components $ 250
Boston Accoustics Pro Series Tweeters $-----
Boston Accoustics Pro Series Crossovers $-----
Boston Accoustics Rally Series 5 1/4" Components $ 160
Subwoofer:
MTX Blue Thunder w/ Thunderform Enclosure $ 90
Power Cables:
Rockford Fosgate Dual Amp Install Kit (4 Gauge) $ 160
Rockford Fosgate insulated hi-grade RCA cable $ 45
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Grand Total (Minus Installation): $3,545
I'm Not trying to brag. Just warning you as to what you can get into. I remember when I had my Jeep. "Hmmm, a CD player would be nice." The beginning of the end.
As anyone who reads this will atest I got KILLER deals on certin things (The MTX Sub and Enclosure, Pro Series). Just remember, BUY now or PAY later. It never hurts to shop around. When I was buying my alarm (Alpine SEC-8063) I was quoted prices from $450 (Not Installed) to Audio Express's $299 (Installed, $20 extra for hookup to the IVA-C800). Shop around. Tell the other stores the deals you've been quoted from other shops. You have nothing to loose but you money.
On a final note, if you take my advice and get a new head unit, get one with pre-amp outs, 4-Volt if you can get them. I had the 2-Volts on my old Sony and the 4-Volts on the Alpine make ALL the diffrence in the world.