Overpowering any device can be fatal to the device and power suplies life. It's always recommended to stay within operating parameters. If you need more out of peltier junctions, I would suggest to use more of them or use ones that are rated for higher power. I found the most drastic increase in effectiveness was by making the insulation better. This old thing I made was 3 coolers off of one power supply connected in parallel.

Have you measured the actual voltage and amps going through the peltier? And are you using one of the 12v rails. I've never tried it with a pc power supply so I'm curious what it would actually draw power wise.

The limitation is most likely the power supply. Peltier coolers have no limitation in what voltage or amperage you can push to them. Too much power and they will fry of course. It may be a 430 watt power supply but thats for all voltage rails combined. Check the label and it should say the max amperage for the 12v rail. I have a 450watt and the 12 volt rail is rated for 16 amps. Under load it only puts out around 11 though. As with most components, pushing them to their max power levels usually will decrease the life span. I don't like buying stuff that burned out so I usually under power everything. So 12 volts at 8 amps has a lot of head room but I'm honestly not sure about the performance difference. You should measure the temp for the current power level and see if there is a big difference if you can get it to the 15 volts at 11 amps.

I don't really know too much about pc power supplies. It could very well fry it. I know I've used wall wart power supplies at varying voltages and amperage in parallel and they didn't fry. But pc power supplies are pretty refined in their output so changing that may have a bad reaction. You may want to ask that in the instructables forum and maybe someone there has tried. A quick google search and it looks like people say it's possible and works but take that at your own risk.

Two liter bottle, Two or 3 peltier coolers would do fine as long as you have the proper insulation for the container. Mine would have worked a lot better if I used something that actually insulated (PVC sucks at that). More coolers will cool quicker but not necessarily get it colder. So how many depends on how quickly you want to cool really. Ventilation is a big part for the hot and cold sides. The hot side for it's obvious reasons. It doesn't have to be well ventilated honestly, but the better cooling for the hot side, the colder the cold side gets. So less ventilation will obviously just be a warmer result. Ventilation for the cold side is also a big deal. At the time, the cold side of my coolers were flat metal plates. This worked...but slow. I added heatsinks later and it cooled much quicker. Heatsinks help the flow of heat by increasing your surface area. So adding heatsinks to the cold side effectively helps pull heat out of the air inside the cooler at a much faster pace. I kept the unit on solid over 24 hrs testing it, never had an issue. I never actually ended up using it because it looked questionable. I dismantled it in an attempt to make a more safe for work look, but it still was not something I'd want to take in to my work place so the project has been scrapped.

I bought it from allelectronics.com
http://www.allelectronics.com/make-a-store/item/PS-575/5-VDC-11A-SWITCHING-POWER-SUPPLY/1.html

But as I looked this up I saw another good looking power supply if you plan to run multiple peltier coolers:
http://www.allelectronics.com/make-a-store/item/PS-1265/12-VDC-5.4-AMP-POWER-SUPPLY/1.html

Most peltier coolers run from about 5 to 15 volts. But just make sure to check the specs on them before so you you don't burn them up.

Yeah. Someone pointed that out today at my work. They told me not to bring in the mini fridge :), might be a bad idea. I guess I should have made it an instructable instead of just a weak slideshow. Oh well, there are plenty of projects left to do out there. Like a plasma gun.

It was initially meant to be used at work but as I finished I could only describe the design as "Questionable". Not one to bring in to work really. The temp readout was going to be the IR thermometer you can see in the background of one of the pics but I didn't want to destroy my only one.