You can use a PCIe 2.0 card, but not a 2.1
PCI Express 2.0
PCI-SIG announced the availability of the PCI Express Base 2.0 specification on 15 January 2007. The PCIe 2.0 standard doubles the transfer rate compared with PCIe 1.0 to 5...
For a 500 Watt System I would suggest at least a 1000 VA UPS.
It will give you enough time to save everything, and log off without rushing.
I like the build and options you have gone with so far. The only things I would change however are as follows.
Why not use a Green instead of a Black WD for your secondary storage drive? Much more reliable and they are cheaper than...