Help to define and determine.

Good time of day! I don't have long watched the progress of the development of computer technology, because I admit that in this matter, I nubyara) Asking for help from knowledgeable people. The problem is the following. I'm not particularly a gamer, mainly only play World of Warcraft. Problem, respectively, describe on the basis of this game. The gameplay itself doesn't cause problems, that is, the movement through the game world, dungeons for 5 people all more or less normal, retarding, but a special role does not play (not at top speed on the medium setting), but as soon as I get to RAID 25 man and it comes to packs of enemy creatures or to the boss, that's ALL!!! wildest lags, freezes for a few seconds, if someone is periodically happens, then I periodically let go for a couple of seconds, that is, the battle lasts 10 minutes, a couple of times for a few seconds unable to move freely. Hope bole mene clearly explained the problem) GTA5 played purely single player game without the incentives, it is acceptable at medium settings, was sometimes freezing, but did not interfere.
So here is the question to understand that the first thing to change in the system unit? The answer, I know) but at the moment I'm only interested in comfortable game in WoW. Iron old a computer like 6-7 years already.

AMD FX-8120 Zambezi 32nm Technology
NVidia GeForce GTX 650 Ti
ASRock 960GM-S3 FX (CPUSocket)
465GB Seagate ST500DM002-1BD142 ATA Device (SATA)
8GB 2-channel DDR3 @ 668 MHz

And another question associated whether I will are limited to replacing only the video card, for example on GIGABYTE GeForce GTX 1050 Ti D5 4G.