First, I want to say I don't use an amp for my interior speakers. I'm stuck half way in this debate between amp/no amp. I hooked an 80x2 up to my 6x9'* and the difference wasn't worth the extra messing around (or money if you don't have an amp laying around). If you absolutely want an amp, though, get a 4 channel. That way you will retain full balance/fade capabitlities. Normally most people don't use these features a whole lot, but they're extremely useful during troubleshooting. I'm not sure which HU you have, but they also sometimes have DSP, so even though you're listening to stereo, there are 4 discrete channels leaving the HU.
If you want to wing it without an amp, it'* easy. If you're not brand concious, and want something that sounds great, try to find 6x9'* and 5.25"ers (or, 4x6 if that'* what you're using) that have 93db (or, higher, if possible) sensitivity ratings. That will give you the most bang for your buck.
Just to let you know how important sensitivity is: Every 3db, it takes twice as much power to drive a speaker and create the same volume. Therefore, A 90db speaker driven with 50 watts rms is just as loud as a 93db speaker driven by 25 watts rms. The higher the sensitivity, the better, obviously, if you're trying to conserve/stretch out your power.
Also, keep in mind that if a speaker is rated for 150 watts RMS, it won't sound that great at 150 watts. In fact, it might not sound good at 90 watts rms. Therefore having a 23x4 rms (or, 50x4 peak) HU doesn't always mean an amp would make it sound that much better (but in every case I would expect a slight improvement).
If you add an amp, make sure it has treble/bass controls, or sounds good. Sometimes, they're only set to amplify the mids and highs, and that wouldn't do you much good. Also, if you want hard hits with your HU and no amp, you WILL need to upgrade the +12 constant and black ground wire to your HU. On a side note, you may be able to fit the amp up front depending on size. Then you'd have limited wire modding and signal loss.
Just like always, it'* best to go to best buy or wherever and give the same speakers a run with and without an amp. Always choose based on how you want it to sound.
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU