Currently I have a notebook running Linux and crunch on this notebook. However, in summer or later I plan to build a desktop PC. At the moment, my desktop runs on Intel integrated GPU and I crunch on Nvidia GPU via a Linux app named Bumblebee. Bumblebee helps to activate and deactivate the second GPU when needed. This way crunching on Nvidia GPU doesn't cause any lag at Intel GPU so my desktop performance is just normal.

Desktop motherboards doesn't have a system like Nvidia Optimus so I wonder can I use the Intel GPU as a desktop GPU and Nvidia ones for crunching? Is there any way for this?
If there is no way, I can buy a cheaper Nvidia GPU for desktop usage. For example a GTX970 for crunching and GTX750TI or even lower for desktop usage. Can I use it this way without issues?

Yes you can. Plug your monitor into the intigrated GPU and set in BIOS to use this GPU on boot up and also enable multi monitor.

You can then run GPUGrid on the Nvidia GPU. You may or may not have to extend desktop.

Wow, that's great. Then if I need some extra GPU power can I run my aplication or game on Nvidia GPU while running on the integrated one, or I'd need to reboot and replug it?

You would have to have your Nvidia card as display card for that so just change in BIOS while your playing game remembering to exit BOINC or suspend project while playing game.

You would have to have your Nvidia card as display card for that so just change in BIOS while your playing game remembering to exit BOINC or suspend project while playing game.

Boinc has a way to do that automatically, I have never used it myself, and it should prevent having to remember to do it all the time. It's in the Boinc Manager under Options, Exclusive Operations in my version. In older versions I think it was under Options, Computing Preferences.

I wonder if it is possible to use primus library (backend of bumblebee) on a desktop computer or does it need special hardware support?

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As I know Nvidia Optimus thing has a little hardware layer named Optimus Copy Engine that transfers display data from Nvidia GPU to IGP and this is in Nvidia GPU side. Additionally, I think BIOS may need to support this setting too. However; I don't know how it or Primus really functions. http://www.nvidia.com/object/LO_optimus_whitepapers.html
Also in the past Bumblebee was using VirtualGL thing instead of Primus. That was sth. different than the Primus, but don't know it a lot too...

http://www.virtualgl.org/About/Introduction

With VirtualGL OpenGL commands and data are redirected to GPU and the rendered frame is copied to the (X)window as image. Additionally image compression can be applied to the image on remote sessions.
It looks like the GPU and IGP can be located on different computers.
If I understand the VirtualGL article correctly, it does not depend on hardware support.
I am afraid of low FPS in games. On the other hand I hit 60 FPS limit in "Minecraft" with both VirtualGL and Primus.
We will see when I get around to building a desktop PC.

For games I don't think it's a limit. FPS rates depend more on the game then compression and transfer to the CPU; I can get FPS starting from twenty to hundreds depending on the title; but I use it fixed to desktop refresh rate of the laptop which is 60.
VirtualGL is server like thing, so can be used for very different purposes. But does the Optimus work on Windows at the same way too? Maybe it has more hardware and Nvidia drivers based structure.