Bob,
    My parents used to own a Plymout Horizon and I'm sorry.  Don't worry, 
along with the abuse you get suggestions.  An Omni is a pretty small car, 
so one 12" ought to do it.  I would suggest a Rockford Punch Pro or a 
Cerwin Vega Stroker, but you are on a budget.  In that case I would 
suggest an Orion DVC XTR 12" and the largest two channel Rockford Fosgate 
amp you can afford.  The Orion has the highest sensativity when wired 
in paralell, and they can take about 500Watts.  The DVC in the spec means 
dual voice coil.  Two 4ohm voice coils actually.  Wire both voice coils 
in paralell, bridge the amp and connect the two.  That ought to pound!  
Most Rockfords come with built in crossovers these days as well, so 
you're in luck.  Oh, the manual will say that the amp isn't 2ohm stable. 
 That's BS, stick around and read peoples posts on Rockford amps.  Many 
people here run their Rockfords (of all kinds I might add) at 2ohms 
bridgeed with no problems.  Those amps are tough and can take the 
punishment.  Also, you'll want to make sure to build the box with 3/4" 
Medium Density Fiberboard (MDF) and to correct internal volume.  Call 
Orion (or whatever woofer you choose) and ask them what box size box this 
speaker should go in for maximum SPL.

    Hmmm... the Orion 12" run about $90-120 on sale, but that amp may be 
out of your budget.  I wouldn't recommend doing this with another amp, 
unless it was rated as a high current amp.  I also wouldn't use a 
Soundstream, even though they have a "high current" mode.  Soundstream 
make some of the best amps in the world, but there are two reasons not to 
use them.  They work best when used for midbass, mids and highs, where 
you need the clarity and they are hella expensive for the big ones you'd 
want for driving subs.  Check out the budget lines that every shop 
carries.  You can use a two channel amp and just wire one channel to each 
voice coil of the Orion.  This puts all the power of the amp into one 
woofer.

      As an aside, I own two Orion XTR DVC 10's.  I run them in a 
series/parallel combination, cause my amp really is only 4ohm stable.  
They get about 50Watts each (peak) and they rock. I don't think they 
pound like you want, but I'm a tweaker not a boomer.  I've head these 
12's in a demo vehicle with lots of power fed to them, trust me, they can 
boom.

     As for the rest of your system, I would use that JVC amp you've got 
and power some co-ax's up front.  That ought to be all you'll need for 
mids and highs, but make sure to get something decent will ya. ;)  I 
would never buy anything smaller than a 6.5".

     What I normally recommend to anyone looking to buy new car audio 
equipment is to educate yourself as to what's out there.  Whip out the 
phonebook and pick at least 5 car audio shops in your area.  Take a bag 
of popcorn and a note pad with you, visit these shops and tell every 
salesman that you see that you don't plan to buy a damn thing today.  
Listen to everything, write down all the information about every 
component that's within your budget and that you would consider buying.  
Then go home and decide what you liked/didn't like and make you 
purchasing decisions away from the shops and the forked tongues of the 
salesfolk.   Salesfolk are the enemy.

Hope some of this help,
Victor Albert