Funny thing is my comp has AMD branding on it but it's ok il live with it. Am I likely to get a good boost with a 980ti?
Probably looking at around a 20-65% performance increase depending on the game.
This site is a great way for comparing benchmarks of popular PC parts just be aware when using it that some pieces can be overclocked heavily by enthusiasts so you will see benchmarks all over the place.
Stick with Anandtechs bench, it's a bit more reliable.
The 980TI is pretty much 1.5 times more powerful than your current card, the only area where a 980TI has less than the 480 is in the size of the memory, that is that the 480 has 8GB of memory and the 980TI comes with 6GB, however the memory bus width on the nvidia card is 384bit instead of 256bit in the 480, meaning that the nvidia card has the edge on the AMD one in terms of memory bandwidth 336.6GB/s compared to 256GB/s the only place this would sting you is in programs like CAD, bitcoin farming basically things that aren't gaming but even then I would think the 980TI's sheer upgrade in every other area would overwrite almost any loses due to having the slightly less memory (unless you were actively looking directly at 8GB worth of textures in game..... which I don't think you could be)
Depends. There is a 4GB variant of the Radeon RX 480.
At the end of the day though, regardless of memory capacity or bandwidth... It's still faster than the RX 480.