-Motherboard seems just fine
-You'll need one x1 to x16 USB riser for 2 graphics cards (one will be plugged in at x16 and one will be plugged in at the riser)
-Processor seems fine
-Hard drive is fine
-RAM is fine
-PSU is useless. Throw it and buy a new one with 1000 watts power supply.
-Those 2 GPU's should work just fine on this built (after the required changes)
-Complete upgrade isn't required
-No, go with 2 GTX 1080
2 gtx 1080 is better than 2 asus strix rog 1080 TI oc version ?
As 1080 is only 8 gb and 1080 ti is 11 gb or my motherboard doesn't support 11gb ?
Even my mining speed will be slow with 8gb when compared to 11 gb ?
I don't think so there'll be much of difference in hashrates between the versions & between 8 GB/11 GB, just make sure it's 1080. If I were you, I'd get the cheapest 1080 available. I've both, RX 480 8 GB and RX 480 4 GB, there's a very slight difference in hashrate (like 7%), while I remember there was a good difference in price. So this memory doesn't matter too much IMO. But yeah, if you're planning to overclock afterwards, you might have to explore more about the versions.
I'll warn you again here though, do NOT go with HP server power supply until and unless you've extremely cheap electricity.
The speed difference between 1080's and 1080ti's is A LOT. In equihash you get 480H/s from a 1080, and 735H/s on a 1080ti.