Yes, it is possible, it is called "bread-boarding" because you use a wood breadboard, cutting board, or even a piece of cardboard, and it is the proper way to begin a build. It is much easier to test a board out of the case with only the memory, video card, and CPU/Heatsink installed, than to get it all mounted, and find it will not boot. It is the basic first step to troubleshooting a build, and the basic first step to prevent you from tearing it all apart if you have a no boot.
And a no boot situation happens an awful lot, especially with new builders!
And I kind of disagree with Outlander_04 a little bit here, because if it works during the bread boarding phase, all you have to remove is the GPU, and then drop the board into the case. No big deal.
If you put it all in the case, and something does not work right, you are going to be handling things a lot more than once. And you may end up having to take everything apart again, and again, and again, I have seen this many times. Especially if it turns out you have mounted the board wrong!
Breadboarding is the best, fastest safest, most fool proof way to start things off, I strongly believe.