System requirements

I have Photoshop CS5 and Silver Efex 3 will not open with error message Not enough memory(ram)
I have a standard Dell optiplex 3020 with 8 MgB of ram

Although 8GB is said to be the minimum, most software will struggle with the bare minimum. DxO recommends 16GB

Can only suggest that you make sure as little as possible is running apart from Photoshop and NIK. This MAY help, but by no means guranteed.

Nor is CS5 recommended, the minimum being CC2020

Here are the Windows Specs

That was a bug that should be fixed in the .8 release