A power question that i have been meening to ask!!

hello all, this is a question about weather my computer is safe to continue to run.

this is my rig (and yes im aware that its in the sig, but may as well re post)

motherboard ex58 ud4p
memory ddr3 1600mhz (not sure what it runs at though when not over clocked)
cpu core 17 920 at stock speed (2.66 i believe)
graphics 2 5870's
hdd 600 odd GB
have apx 6-8 120mm fans running.

power supply is 850w from thermaltake (toughpower)

over clocked, I run the CPU at 3.7GHZ and Memory at apx 1500mhz

is my power supply safe to use or should i upgrade it to a higher power supply. computer runs quite hot and heard that insufficient power can cause heat. also noticed that the fans slow down some times (RPM sometimes drop)

supply is about 2 years old

edit: graphics cards are both highly over clocked when in use. graphic corruption can sometimes be noticed, WHEN NOT OVERCLOCKED!!!! and moderately clocked. nothing 2 bad, just little white and green squares every now and then.

also use quite a few USB items, like HDD and g35 headset.