what else will i have to upgrade?

How much are you growing your system in watts?  That will give you an idea if you are getting too much for the alternator in use now.  Will you be running the amps at 2 ohms or 4 ohms?  2 ohms will need more current, provided by the alt.  You will also need to look at all the wiring and fusing in place and make sure you have sufficient gauge for the equipment.  A fresh battery is never a bad idea, too, but if you already have a new one it should be fine.  All it has to do is start the car and take a charge. 

And if you haven't upgraded the wiring in the engine bay now is the time to do that first...battery ground-to-chassis, alternator power-to-battery, chassis-to-engine block - with battery ground-to-chassis foremost on the list.  You can add new larger gauge wiring to what is currently existing (use a gauge as large as your system's main power and ground wires).  Wire-brush contact points in all locations.

So, you do the new wiring, do the engine wiring upgrades, clean and wire brush contact points....and hook up the new sytem.  If then you are having trouble getting enough power output from the alternator, you upgrade that.

There are two definitions of The Big 3:  upgrade the wires as I described above, and the 2nd is that the big 3 are wiring, alternator, and battery.  The second def adds more to the first def.  That means that to upgrade properly, step by step, you do the wiring upgrades first, including the new larger gauge wiring that will power and ground your new equipment.  After that, if needed, comes the battery and alt.

Like I said, you do that, you install the system, you don't worry until it's time to worry.  At that point, if the current powerplant isn't capable (and I agree with cyber that it's very likely you'll need an alternator upgrade), you upgrade the alt.  At that point you can take the car to your local auto shop and have it load tested.  The electrical system will put a demand on the powerplant and that demand can be read with the proper tools.  You'll know at once what you need to get, if in fact you need to get anything.

Appreciate the thank you!...Wiring:  most amplifiers will specify a gauge to use for the amp itself;  I would say 8 gauge for each amplifier.  The main power wire needs to be big enough to carry the maximum amount of amperage the combined amplifiers MAY pull all together.  Figure that by looking at the fuse rating of each amp and combining them.  So if you have three amplifiers who each have fuses valued at 60 amps each, you will need a power wire that can carry 180 amps.

Look at your amps' specs to determine, then use Power and Ground Chart to determine the correct gauge to use.  Buy enough so that you can install it comfortably to the install location where you should place a fused distribution block.  The distro block should have at least one input at the size of the main power wire and enough outputs for the wires leading to the amps.  The fuses in the distro should match the rating for those wires.

The power wire, too, needs a fuse close to the battery with an appropriate fuse holder.  Use the chart again to determine that fuse size.  Grounding can be done from each amplifier to chassis, or a neater install is to use an unfused distro block and one main ground wire that matches the gauge of the power wire.

http://www.crutchfieldadvisor.com/S-MS1zsuJlU01/learningcenter/car/ has some good basic info on installing amplifiers.

For battery recommendations from this forum, do a search  for "battery" under "topic".  Me, I use a stock battery replacement when I need to change it.  But I don't sit around playing the system a lot with the car off. 

1. (and I'll say this and get it out of the way...no elaboration...)  LOOK at the Sonys - LISTEN to several other subs.

2.  Whatever the output rating of an amp is, is what it is no matter how many drivers are connected to it.  The 700 watts output will not double because you have two subs hooked up.  Half would go to one sub and the other half to the other sub.

3.  DVC 4 ohm sub, coils in parallel for a 2 ohm load, onto a mono amp capable of driving the sub at or under the sub's RMS.

4.  Two SVC 4 ohm subs, wired together to make a 2 ohm load, both driven by one mono amp capable...as above.  You should easily find one mono amp that will power what you want to drive...500 to 700 watts at 2 ohms.  The benefit of using one amp instead of two is:  less power draw from your car, smaller footprint, simpler setup, less electronics, just as much music.

5.  Wiring configurations that will help you put subs together of varying voice coil configurations.  A mono amp is the smartest choice (some of the reasons listed in 4) for subs over a two-channel amp.  IF you are building for sound quality, which I presume you are not as you are looking at Sony, you would use 4 ohms as the load and be content with a lower level of sub bass.  Normally, now with the advent of 1 channel mono amps for subs, 2 ohm loads are the choice and you will be able to achieve a greater level of db without increasing size or cost of amplifiers.

Here's a link to a Big 3 upgrade thread with pics:  http://forum.sounddomain.com/ubb/ultimatebb.php?ubb=get_topic;f=5;t=007801