I have not seen this problem. It seems to be saying you don't have enough disk space, but you do. You really can't proceed without enough disk space recognized by Windows Install. So, what's the hangup??
It's not your power supply, that's fine. It is remotely possible that you disabled your disk drives in BIOS or in Device Manager, but I highly doubt that. You can verify the disk drives are there by going into BIOS Setup (click F2 continuously when you power on). You should see the two drives. Go to Device Manager and check both hard drives are shown, and enabled. I don't think that's the problem, but I'm suggesting checking every possibility for trouble.
You must have a 64 bit OS to access more than 4GB. so you must install Win 10 64. It turns out Windows 32 and 64 are very different and you can't use Windows Upgrade to move from 32 to 64, the OS kernel is too different. (You can use a transfer program from a third party, such as PC Upgrade Assistant from Laplink Software, to upgrade in place, but that is another topic.) So typically, you must do a fresh install. Back up your data and get all your software disks ready to reload them.
When you have a problem like this, not seeing your hard drives, I suggest a clean install with known good source. I am not sure I trust the Windows 10 DVD you have, so I'd get a new one.
Get it here:
https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Dsoftware&field-keywords=windows+10+recovery&rh=n%3A229534%2Ck%3Awindows+10+recovery
Or get it here:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR0.TRC0.H0.Xwindows+10+recovery+disk.TRS1&_nkw=windows+10+recovery+disk&_sacat=0
You must have a key to activate Windows, and I will leave that for you to find.
Once you have Windows 10 64 loaded, you need to get your graphics working. Your existing Windows loaded the driver for the standard VGA graphics on the motherboard and enabled it. So video is available from the VGA connector on the back. Check Device manager, maybe it will show Standard VGA or something like that, and enabled.
When you installed your new graphics card, perhaps it did not get a driver loaded for it, or it did not get enabled in Device Manager. So make sure you load the driver, then go to Device Manager and disable the motherboard graphics and enable your new one.
As it happens I am running a 960 on this PC. Love it, no issues. It's way more than I need, but so fun. I had to connect two power cables to it. It really uses 12v power, my guess is about 15A. That should not be a problem with your Corsair CX500. Be sure to connect all the power connectors though, to avoid voltage drop in the cheap cables and connectors on PCs.