@OP Increasing the PSU won't change things unless your matching it to a new required card, which honestly except for SLI you don't need anything above 600W normally. SLI can help improve but so few games actually 'customize' for SLI (in fact the latest drivers from Nvidia were noted as having NOTHING for SLI profiles yet they are the 'Titanfall' drivers.... *_* ). I don't go SLI simply because the nextgen of cards (already out and now massively produces, the 8xx series) usually is more cost effective then buying two SLI cards and performs as / better then the SLI combination.
Honestly try some basics first:
Download and run SPECCY, copy and paste the first tab to show your idle temps
Download and run MSI Afterburner, run some of the games that crash, what temps are you getting when underload?
Open Computer, Can Air dust out the bunnies and use paint brush on the vents, coolers, fans, etc.
Did you install all Windows Updates? Including options except BING? Check them and repeat till ALL are installed.
Download and run Slim Drivers, install all the latest updates but you don't need to reboot until you do the last update
Remove whatever AV your using and go to www.filehippo.com and download AVAST! or AVG and do a full system scan - this repeatedly has resolved alot of people issue relying on MS Essentials.
Download Malwarebytes do a full system scan (AV doesn't pick up alot of malware) - this resolved almost ALL other similiar posts to date as most had Malware the AV didn't pick up.
Repeat the AV/Malware scans till the system comes up clean.