Do i need an amp?

I just installed a JVC 40wx4 head unit left over from my truck, which is no long with us. anyways, i also pulled a pair of 5"x7" speakers from the truck that have 120w output each. I'm doing the best with what i have, so i'm not trying to do anythign special.
I just want to put the 5x7's in the back of my new car, which have speakers that only have 20w output. Do i need an amp to run the new speakers i want to install properly?
I know when they were in the truck they sucked alot of power from the head unit, and the HC would often shut off after a certain volume level.
So yeah, i'm just wondering if i need an small amp to put in these speakers and play them the way i want to.
any info would be appreciated, thx
-godisoutofthebox

First thing, the speaker is rated for input, not output. Second thing, I doubt the speakers are drawing so much power, that it is killing your headunit, unless something was wired wrong. How many speakers were hooked up to the head unit?

only four, some smaller ones up front that i didn't even bother with taking out of the truck.
So what you're saying is that since it's rated for input that i can hook the 5x7's up and be fine on power? I don't expect to play them at full blast very often, if that, but i would like to it to have a respectible sound quality. So if I only have 40w going into the each speaker, how does that affect my sound?

if you aren't going to blast them they should sound OK, but don't expect anything. also, i did something similar, and ended up ruining my speakers, but they were shitty anyway. Now I have some pioneer 5 1/4" AND a sub all hooked up to the HU (ill have the amp hooked up soon) and they do pretty well for what they have.

Personally I would put in an amp if you can get a deal on one. Youd be supprised how much difference an amp can make...kinda depends on your speakers/deck though too....Ive put in alot of decks and speakers into cars and the majority of them dont have amp and they sound ok. Its all depends on what you want.

Today I just finished putting the speakers in w/o an amp. And after an afternoon of trying to jerryrig my speakers into where the old ones where w/o it looking ghetto, the sound is very good for what i want to do. Just some nice sounding speakers that isn't too loud, but gives me what i want when i do want to have alittle bit of volume. I think i'm going to spend the money on what i would have spent on an amp, on a pair of new rotor and pads for my front brakes. Should be interesting, seeing as i've never done it before, but i should be able to figure it out, i've got a chiltons to help. Thx for the advice, maybe if i want somethign more out of my car, i'll get an amp, and maybe i'll pick up a set of 10" subs from my friend who just got new 15's. but until then, what i've got is good. thx again for the advice.
-godisoutofthebox-

HA.. good luck with the chiltons. Ive said this before, but in my opinion chiltons are useless with a capital USELESS. Thats just my opinion on chiltons. But Im glad you got your stereo hooked up right.

lol, well...it was either that or trying to do it w/o any clue of what i was doing. Besides it was like 7 dollars at bookmans, so what little help it provides me wont be money wasted.