I've found ebay pretty good for second hand amps. Recently bought a Rockford 800a4 for £200 - that's a pretty powerful amp that originally retailed around the £500 mark. Stick to the big names and you can't go far wrong. My rule of thumb for prices of second hand stuff is about 50% of original retail.

As for speakers, it's definitely worth changing the stock ones ! Infinity & Focal seem to be the current vogue, all depends on your budget really. I'm assuming you're in the UK, if so InCarExpress come recommended.

Steve.