Let me break down for you what you need to understand when looking for PC parts.
CPU: Central processing unit. Responsible for rendering. The more cores and speed, the less waiting around you have to do. Also note that instructions and L# cache can make a big difference in the effectiveness of a CPU in rendering. Always make sure you are buying the current generation of processors.
GPU: Graphics processing unit. The use of this depends on the program you are using. With some programs this will accelerate the rendering speeds of your videos. Without one, you will not be able to view your videos in 4k because intel HD graphics does not support such resolution* You will also need this for tri-monitor set ups. There are two different types of GPU's: consumer grade and professional. Consumer grade is the everyday cards you see. Ex, GTX 760. Professional is the quadro cards made for mad scientists and Hollywood film enthusiasts. They will range from $400-4000. If you ever intend to do any sort of gaming ever on your PC, do not get the Quadro's! They are not officially supported. 256 bit bus is twice as fast as 128 bit bus. The higher RAM, the better. More ram is better for more monitors. Example, if you are only using one monitor, 256 bit bus with 1gb ram is a better option than 128bit bus with 2gb ram, which would be better for tri-monitor (assuming they are same price.) CUDA is a technology developed by Nvidia to assist in rendering, while its counterpart is Open-CL, developed by AMD's ATI. The choice depends on which video editing program you seek to use.
STORAGE: Hard drives, SSD's... RAM drives... Everybody loves lots of storage, but what about performance? The common trend gamers and video editors are following is this: SSD drive dedicated for OS and editing programs and a HDD for storing video. SSD's come in all shapes and sizes (not really, they are all 2.5") and only some brands are trusted to not fail and keep their promised speeds of 5x faster than a HDD. The most notorious brand for SSD's is Samsung and arguably Intel. Samsung lead the revolution of SSD's with its extremely low failure rate. Intel and other companies have adopted similar technology to make SSD's a great alternative. Some other brands that are trusted besides Samsung and Intel are: Mushkin, Crucial, Kingston, Sillicone Power. Samsung and Intel are the fastest, though. So let me break it down.
SSD's: Very fast performance. It's 2015, you must own one, or else you are... an idiot. I bought a Intel 730 240gb and I have all my editing programs + 10 games and it is getting full (80%) and has started to slow down the write speeds but not the read, and I could fix this by clearing up some storage.
HDD's: Only get 7200 RPM. WD and Seagate are the most common brands. If you are on a budget, get the WD Blue. It is cheap and it works. For more reliability, get the HGST deskstar. It is high performance WD drive.
RAID: The most important term for you to understand with HDD's is RAID. RAID, or striping, is extremely important when working with 4k, 3d or high resolution material. RAID 0 removes the virtual barrier between two HDD's so that you may double the speeds of two HDD's and combine their storage into one. If you buy any Z97 or x99 motherboard, I can assure you it has RAID built inside of it. All you would have to do is enable it in the BIOS and BAM, your drives just doubled in speed! It is not a bad idea to buy two 1TB drives to make your own 2TB 14000 RPM drive.
Motherboards: There is a common misconception that motherboards can become a "bottleneck" in the performance of your system. This is not true: motherboards will give you the same output for your CPU no matter what. The only difference is things like PCIe lanes and other bells and whistles which are optional. Example, two graphics cards slots, on board audio, PCIe lanes, wifi, and other such things. For video editing, I would get one with good on board sound, or be prepared to add a sound card if you want to make sure your sound is premium rather than just great.
RAM: Random access memory. This will determine the amount of pre rendered footage available for preview in After Effects and other programs. It will not effect rendering speeds, and is a great commodity when needed, although not crucial. 16gb is good, 8gb is limiting.
PSU: This is the most misunderstood conundrum in computing because people think that the only thing that matters is the efficiency rating. Sure, saving 5% in power can be a great way to help save the environment (and your wallet) but ultimately that $20 a year you save in power because you went Gold over Bronze in the 80+ efficiency rating won't mean a damn thing if it starts popping silicone popcorn and burns your house down. In fact, power supplies are so important this guy called jonnyguru gets paid to review them for a living. Some notable brands for power supplies are Seasonic, and xFx. There is a team of japanese that make the best power supplies in the world. Their name is not ringing a bell right now, but I would get one of the power supplies they engineered. You can find the best power supplies on jonnyguru.com
**NEWS**
Intel: New 5th gen codenamed Broadwell CPU's expected to launch sometime in Q2 this year. It might be worth the wait as these will be slightly faster (10% at most) but the 5th gen price will replace the 4th gen.
AMD: Not doing nothing new. Same old architecture, 32nm is a joke compared to Intel's 12nm. They are surviving. Expect their stock to tank. Their server chipset fell through and now we should all be concerned because Intel is gaining a monopoly on the market and we will be forced to pay premium. Although I do appreciate the price of the 5820k.
That's it for now. A lot of information is missing, but I did hopefully enlighten you on the basics for video editing.