Use an Intel CPU, I would not use an AMD for gaming.
ASRock motherboards are brilliant for the price, stick with them.
8gb RAM is a minimum, 12 or 16 would be better. 1600MHz used to be the recommended max for gaming but these days the price difference between DDR3 1600 and DDR3 2133 is minimal so I would go the 2133MHz.
The monitor is not good enough for gaming. 1920x1080 minimum with a response time at least 5ms unless its a IPS then 10ms is acceptable. I gamed for 3 years using 1680x1050 and the move to a good quality 1920x1080 was a HUGE improvement! Now 2560x1440 is the norm and soon 3840x2160 4K will be the new ultimate!
4gb graphics memory is fine with 1920x1080, but some games are approaching 6gb at 1080p, ridiculous but true!
I prefer Nvidia graphics, but understand that Radeon is often cheaper for the performance, albeit horrendously inefficient!
850W power supply is overkill for a single card. I'm planning on running Titan X or GTX 980Ti SLI overclocked on a 750w power supply and I'm confident it will run fine.
I used to run SLI for years and reaped the benefits as it is the cheaper than single card performance, although many on here disagree, simply they are wrong! As you are after medium gaming performance a single card is your best option.
Go over budget to get an SSD, once you have had one you will never turn back. I'm on my 3rd (120gb, 256gb, 512gb) and keep upgrading as I run out of space. the 512gb has been good. 150gb free with 20 games installed.
For a gaming rig its not necessary to have a permanent disk drive, I use a cheap Samsung portable drive to free up space in the case. Personal preference though.
I have a GTX 780Ti that I will be selling when I know more about the 980Ti. If your interested let me know, it is a better card than the R9 290.