Speaking from personal experience. If you're going for new hardware setup, here's my advice:
1. CPU - i7 5820k is VERY effective, you don't have to worry about overclocking it but if you can overclock it to 4.5GhZ or something of that sort then you're pretty much sorted till the next generation of gaming.
2. RAM - Not much to look in here, if you're looking for 20-25 characters then you're going to need at least 32GB of ram, DDR4 2400 MhZ (Preferably)
3. GPU - If you ask me, GTX 980 TI is pretty expensive but well worth it, if you can afford to spend $600-$700 on it then go for it, it's well worth it. However, if you're short on budget after the RAM and CPU, I recommend GTX 970, preferably GeForce Strix, (To be honest, get a SLI of that and you're pretty much good to go, SLI GTX 970 is better than a 980 Ti and way more efficient) If you want a little bit cheaper version of Strix, MSI GTX 970 Gaming is beautiful, I have it and I absolutely love it. I run 15 WoD accounts and in big battles my VRAM usage is usually 2500 MB, 60% Core usage.
Good luck and be careful with hardware, do your research very well before you decide to buy anything. Don't take my recommendations for granted, do the research first and if you're well enough convinced of my recommendations after your research then by all means, go for it and have fun gaming!