Thank you guys for feedback so far.
I usually shoot 2 things:
1) landscapes etc. on holiday
2) people close to me
So, I am currently learning about portrait photography so that is why I got the 50mm 1.4 as everyone told me its a great lense. I am hesitating whether or not to get the flash SB-900.
When I shoot nature the 18-105 is great as I have a freedom to position it quite well but still my photography skill is a beginner but I want to keep improving. I won't be professional photographer as this is just a hobby but that doesn't meant that I don't wanna be good. I work in animation field so this also helps me improving my staging etc.
I'm also toying with the idea to try macro photography of insects, flowers and all the "little" life that we miss.
Based on what you've said above, I honestly think buying a D800 would be crazy & a waste of money for the time being. The reason I say this is that it WILL NOT help you take better photographs. You say yourself that you are still a beginner, so what you need is not new gear - it is practice! Take the time to learn about light and composition and your D90 will be able to produce photographs that blow your mind.
So, I've had D90 for almost 4 years now and I started editing photos in Aperture almost 2 years ago. The reason why I am thinking of upgrading is that I know the D800 is super great body and the videos I've seen from my friends are superb. The still quality is amazing and I thought that its better to do jumps then little steps. ie. if I buy 7100 now and then D800 later then I might lose more money overall then if I bite the bullet now and go straight to D800, don't you think?
No. I think it's better to build up your kit gradually, only upgrading when you have reached the very limits of what your kit can do. Blowing £1800 on a body you don't need will hinder your photography rather than help it, because you might not be able to afford other things that would actually help you take better photos.
I have not seen your friends' photographs of videos, but I am absolutely certain that what you like about then has nothing at all to do with the camera that captured them. If your friends are more experienced than you then it's far more likely that the shots are well composed and this is what appeals to you.
Video is mainly for references as I sometimes study details of movement (my job

)) ) but I'm thinking to do a short movie with my gf as we already have a concept etc.
So, that is why I said the video is not super important but its somehow important. Still quality is mainly what I'm after.
Have you looked at dedicated video cameras for this? I don't know much about video, but I believe Sony make some great cameras that will be far cheaper than a D800. Even if you do want to shoot video on an SLR, there are many other options that could save you a lot of money as well - consider something like a D5200 (they're under £500!).
When it comes to lenses, the 50mm and 18-105 is usually what worked for me although sometimes I wished I had something that goes to 200mm or even 300mm (things like ZOO or nature when you see beautiful deer but your range doesn't do it)
Now, a side topic. Do you think the SB-900 is enough for decent portrait photography? I noticed that you can put it on a stand and position it etc. so its a good compromise as I can have it with me when I'm out but also can position it in a room when I'm at home and playing with a "model"
The 50mm is a great lens & will be good for the portraiture you want to shoot. If you'd like a longer lens, I've heard great things about the 70-300 - it's supposed to be very sharp.
The SB910 is an amazing flash. It will do everything you need an much more. Spend some time learning about lighting and it will definitely pay off.
Please take a look at my Flickr page - there's a link in my signature. I'm not saying this to promote myself, but the photos on there are mainly portraits and nearly all taken on a D90 with a 85mm AF-D lens (£300 new) and either one or 2 SB24 flashes (about £70 each, second-hand). Not expensive gear, but all very capable equipment.
As a beginner it's natural to want to improve quickly (every photographer has been there) and "gear-lust" grips us all from time to time. But rather than spending money on new equipment, just learn more about the areas of photography that interest you. There is no magic bullet and it's not a quick process, but as you learn more about it all you will start to see that the equipment has far less impact on the final image than you think.
Sorry for the super-long reply, but I really hope that helps.