I was trying to keep the budget down for my build, got a great deal on them and decided to go with some Infinity Kappa's on my car but there are a lot better/nicer speakers out there depending on your budget. You might try starting off by looking up some specs online and then match up a few speakers to your available "true" power level that you'll have to feed them. By true power be wary of the "generous" 50x4, 60x4 watts, etc. ratings you might get from a head unit manufacturer (if that's what you plan to continue using). Also watch out for speakers rated to handle way higher power then they really can, so pay attention to the minimum suggested power ratings more than peak numbers. Then look at other things like cone and surround material, frequency response, etc., but most importantly go to a real stereo store (preferably not Best Buy, Circuit City, etc.) and take some of your CD's with you and sample a few different speakers. Everyone's ear is going to be different on what they like.
There's one other thing to note if you still remember how the stock head unit and speakers sounded and are using that as a point of reference. They can come across as very punchy and I know many people that aren't really into audio who think it's a great setup. And for a stock system it isn't too bad, but the equalization curve they apply for the speakers is mostly responsible for this and not quality of speakers and head unit power. With just the upgraded front components and rear 6x9's (i.e. no sub connected) you might have thought I'd lost bass response if you got in my car, even with the small external amp, but start cranking it up and the stock setup would have started clipping and producing some nastiness when the aftermarket setup starts to shine. Like I already mentioned the head unit output/EQ can make a big difference as well and swapping different HU's that I have can cause some noticeable changes in overall sound reproduction on my setup.
Eric H. - '04 Neon SRT-4 (eBlue/S2) - '04 SRT-4 (blk/stock)
'92 Dodge Daytona IROC R/T --
'91 Dodge Spirit R/T
plus a couple other turbo Dodges, a Cobalt, a couple Focus, a V-wagon, and some other stuff...