If it were my money going in to a Dodge...
I would go with a 10" Type R sub and MRD-M605 amp combo, and spend the remaining on the box and shipping (from
www.onlinecarstereo.com)
With a little more I would get a 10" image dynamics IDQ, and a PPI DCX 300.2 amp.
If I recall your truck has very little room for subs and stuff, so you may want to check out JL audio. JL has "stealth" boxes wich fit Dodge trucks, then it's just a matter of powering it.
One more thing you should also include in your budget wires, if you keep a stock system chances are you'll need line converter (I think thats what they're called) to take the signal from the truck's speakers and into the amp, and unless you're installing this your self then you should also include labour. Keeping all that in mind you may find your choices that fit your budget limit themselves.
Good Luck, and don't buy crap...