what should I know about GPU'S and Hardware and also I need someone to compare 2 GPU'S for me

[XpZxp gaming]

Hey I want to upgrade my GPU and also to learn how to build a pc. Honestly, I have no idea where to begin with, there is so much stuff to learn. From clocking to what each letter mean in GPU and all the other things like motherboards memory and all that stuff. so, where can I learn that from? How much time will it take? and is it worth it? Also I will be glad if someone will do a comparison between 2 GPU'S I can choose from the GPU'S are:EVGA GeForce GTX 970 4GB SC GAMING ACX 2.0, 26% Cooler and 36% Quieter Cooling Graphics Card 04G-P4-2974-KR
and:Gigabyte GV-R939G1 GAMING-8GD G1 Gaming Graphics Card AMD R9 390 512 Bit GDDR5 8GB 2xDVI/HDMI/3xDP I personally like NVIDIA but this is pretty much good price and I believe the price will be higher in NVIDIA for the same preformence
ps: When I talked about buying GPU for my pc with some shop they said I will need a PSU additionally is this true?

 

[hang-the-9]

How much time it will take to learn, basics you can pick up in a day. There are many beginner guides to PC hardware out there, just do a web search.

Sorting out what is better, what the parts are, etc.. is constant since there are not only new products that come out but often there are old products updated with new product names that don't really change aside from the name.

Go figure out what card is good for your PC you need to look at power supply, case size, motherboard layout since you need to actually fit the card in your case. You can't take a cheap basic use system in a small form case and just stick a GTX 970 or a R9 390 in the thing. Also if you have a slow CPU either of the cards won't do you much good.

Read this and fill in some more details http://www.tomshardware.com/forum/290969-33-advice-choosing