Motherboard Replacement Advice

Have you filed a support ticket with Gigabyte?

unplug everything.

Check for bad capacitors on the board (bulges rounded tops liquid of something oozing or pussing out at the top or base, odd burning chemical kind of smells are major indicators popping noises no matter how quiet or surging noises brown outs zzzzzzz noises or sizzling or a constant-at any point any time or high frequency/pitch whine) as well as whatever you can see inside the power supply. if the warranty is up on the power supply take the cover off and inspect the internals for the same. (if you see any white goo or yellow goo looking spots make sure the power is turned off and use a PLASTIC knife or fork to test if the goo is hard in which case it is just excess glue, DO NOT stick anything metal in there as the capacitors still hold some current and will shock you if you touch the wrong spot)

if you smell/hear/see something funny it's a guarantee something is burning out.
from my personal experience if the 5volt rail on the psu or the 5volt regulators on the m/b is burning out it will take out the usb (which would explain why your 2.0 is gone but your 3.0 is still working because the 3.0 uses a bit more power) and i believe that is the same rail/regulator handling the 3.3V memory power as well at some point where the power is divirged/splitoff. so if your getting power surges or power browning from say the electrical companies offpeak power saving boxes (those cause alot of light bulbs to burn out from all the dimming when the off peak kicks in which combined with issues of Vdroop around .5v {one report of .75V vdroop was in a review for this m/b} for your board would possibly cause surges to burn out the regulator when the offpeak boxes shut off)
if you have a newer power supply tester get the psu checked out asap

there's enough bad reviews on newegg on the 850w coolmax power supply in 2010 if that's about when you bought this one that i'd be real leary about trusting it and has made me highly suspect this psu is the cause of your problem as there were similar reviews on bad psu's i have bought or customers pc's i fixed:

turks
10/24/2010
Pros: If you are on budget and slowly acquiring parts to make a PC, please test this device within 30 days of purchase from Newegg. Power supply failed due to low output voltage. Unable to completely power motherboard and other devices.

Cons: Since these is 3 years warranty manufacturer is the only option for replacement. Hope Coolmax expedite replacement.

K
6/26/2010
Other Thoughts: when I bought this there were many poor reviews. I took a chance on a daily deal and have been happy since.
AMD X4 955 Black OC to 3.8GHZ, Kingston Ram, (2) ATI 4850's in Crossfire mode, (2) Seagate 500GB drives in Raid 0.

N/A
3/25/2010
Pros: Essentially none.

Cons: Worked for about 5 hours, then began cutting out. In those 5 hours it also performed worse than the 500 watt I was replacing it with. Just avoid this altogether, buy something better.

essentially your psu should be giving you more than enough power, but if they used shoddy parts anywhere in it it doesn't matter. 66amps is pretty good with all you are running it's more than plenty also.

Well Coolmax is on the avoid list and not because of the size rather dirty power that can cause all kinds of issue. Pretty sure Gigabyte would blame it on the PSU.