Insufficient Power Supply

I would have thought that 18A is off each PCI-E connector (36A combined) if it's 600W system.
Mainly it depends on the quality of the PSU, good quality ones have much better efficiency ratings. I'm running my GTX260 on a corsair VX450W without any issues at all.

If the PSU wasn't delivering enough juice the PC would crash/shut down. I suspect that other issues are occuring here. Using the latest drivers from Nvidia?

If you provide full system specs we might be able to work out what going on...

You have made the right choice to change the PSU whatever happens, Coolermaster make poor units which have been shown not to deliver their rated wattage and current.
Yes, it is possible to damage the system if it is run underpowered and yes it may very well underclock under those circumstances although it`s just as likely to crash while running or simply fail to boot.

You don't combine the voltage - you combine the amperage on both rails. So an 2 rails, e.g. 18A + 18A, gives you 36A total input for you're GTX, but the voltage is still 12V for both rails - this effect doesn't add up because of the way voltage (electrical tension) works. If you know you will be running 36A on 2 rails you need to look at the max amperage rating for all of the 12V rails. Then you need to know if the 36A + the amount of amps you CPU and anything else on other 12V rails exceeds the max total amperage rating (e.g. if the max total rating for all 4 12V rails is 41A:
41A -36A = 5A, then you only have 5A left for you're CPU and anything else running on a 12V rail before you max out)

As for the 850TX it is a brilliant PSU, a bit overkill for the system its running but plenty of room to upgrade - you could probably SLI with 2 GTX 260's in the future (kinda guesstimating there)

I installed the psu for him today. That was actually the problem. The game occasionally dips below 50 fps now, but not often.