It's not overly surprising that a cheaper power supply popped after running at full load continuously for 2 hours. You can see by looking at this link of 80 PLUS ratings that 50% of rated load gives you optimal efficiency.
http://en.wikipedia.org/wiki/80_PLUS#Efficiency_level_certificationsRunning a power supply beyond 80-85% of it's max rating continuously is inviting trouble. Here's what I would do. Add up the power consumed by your GPU cards and CPU, then round up a bit to account for the motherboard, RAM and fans. Then add 2 zeros onto the end of that figure and divide by 50, 60, 70, 80, whichever percentage of efficiency you'd like to achieve with your power supply. That will tell you how many watts you need.
Here's a chart I downloaded from somewhere that shows how much power the various GPU cards require, in addition to megahash rates, etc...
http://i93.photobucket.com/albums/l59/lytlesnake/VhgVh.gifI think the (80 PLUS/Bronze/Silver/Gold) power supplies are great if you want to spend the money. But remember, you can also apply the 50% Max Efficiency rule to cheaper power supplies, which is probably ideal if you're trying to build an overall cost effective miner. By overall cost effective, I mean striking a balance between initial cost and electrical efficiency. Then again, probably a majority of modular PSU on the market have an 80 PLUS rating at this point anyway.
