Almost all amps will sound better than head unit powered speakers... Head units put out 25Wx4 or less RMS... almost all amplifiers put out more power...
Almost all non-woofer car speakers are 4 ohms... when you get into subs, there are wacky type wiring configurations, crazy ohm loads and such like the guy in that line was doing... Assuming that you have 4 speakers and no subwoofer, you probably just need a standard 4 channel amp amplifier... not sure what speakers you have, but check the specs for the max power/rms power and get an amp in the same power range... if those speakers sounded good on head unit power, you can probably get like a 35Wx4 RMS amp and have it sound better/get louder w/less distortion
Here's the ebay listings for multichannel amps...
Alpine 4-channel Amps
Best way to figure out how certain amps sound is to go to a car audio store with your model of speakers or specs similar to your speakers and try the different amps with it (Cartoys usually doesn't mind if you try out their demo room stuff)... then buy the amp you like online
Personally, I'd recommend you with some of the more "Name" brands and buy from an authorized dealer (for the warrenty)...
Cardomain always has crazy deals going on:
I got my Phoenix Gold Ti500.4 from them for less (~$270) than I could have gotten it for on eBay (~$350), plus it has the full manufacturers warrenty since Cardomain is an authorized dealer.