Equalizer's and Crossover's
 

First off, I've got:
Sony Head unit
Sony 12disc changer
2 12' Jl Audio Subs ran off Majestic 500watt bridged amp 2 chan
2 6x9 Kenwood 3ways off 4 chan600wattPowerAcoustic ran by1ch


What are the benefits of having a crossover or equalizer?
Which should I get first? Or should I get Both?

Get all your speakers crossed over. This will maximize the efficiency of both the speakers and amps, as well as give you the biggest boost in sound quality by cutting out distortion.

What a crossover does is it limits the signal going to the speakers. You don't want the same signal that goes to your mid/high speakers going to your subs, just like you don't want the same signal that goes to your subs going to your tweeters. There isn't any one speaker out there designed to play all sound frequencies. Crossovers eliminate that problem. You don't need an EQ, its nice to have if you know how to properly use it, but it is not needed most of the time.
J

You might want to check, but some new amps have Crossovers built in... the new MTX Thunder series for example

Most all new amps have built in x-overs. The vast majority of the ones that don't are at least 4 years old.
J