(More just answering the RAM question more than anything - others will be better suited to suggest rigs for your usage if you post a budget and what kind of rig you want.)
Effectively, the 1600MHz is how many clock cycles per second the RAM can achieve.
So 1Hz = 1 clock cycle per second
So 1600MHz = 1,600,000,000 clock cycles per second (as it is Mega Hz, not Hz).
In more basic terms, that means the RAM can transfer that many pieces of data per second - so theoretically, the higher this speed, the faster data can be transferred (there is more to it, just putting it simply).
As for the 1x8GB vs 2x4GB - without going into much detail, I like to imagine it as if the data (8GB) from the RAM are
"cars" trying to get to a destination, and the RAM slot (x2 or x1) being your
road.
If you put it simply in these terms:
1x8GB means you have 8GB worth of "cars" travelling through one road - it can typically mean the time it takes for everyone to get to it's destination, is slower.
2x4GB means that you have 2 lanes of traffic, where "cars" can travel down either lane, meaning it will take less time for everyone to reach it's destination.
Realistically, there is no exact science, one thing for sure, is you always want to opt for dual channel over single channel (I.e. 2x4 over 1x8) as you get the "2 lanes of traffic".
Next to that, it can depend on the application you are using, the technology you are using, and what the rest of your system is - for example:
- If you are just running DDR3, speed won't really make much of a difference
- If you're running a Ryzen rig (with DDR4), then higher speeds are preferred as Ryzen uses higher speeds better.
Bear in mind DDR means double data rate, so technically your RAM module may be 800MHz, but at double data rate = 1600MHz.
Lastly, you can't interchange DDR3 and DDR4, only DDR3 boards can use DDR3, and only DDR4 boards can use DDR4.
DDR4 is the latest widely available technology and is the format of nearly all new motherboards.
You'll want to look more into CPU and GPU for CS, by no one will be able to predict exactly what FPS you will achieve, as this is also dictated by your monitor and what settings you run at. You may want to post your budget and specs you had in mind for others to recommend good builds.