Have fun paying hundreds of dollars every few months just so you can keep up to date with the current hardware standards for PC games then.
Upgrading the PC every few months gives you little to no benefits, thus it's pointless. No-one who knows anything about computers would ever do that. If you choose the hardware wisely, then the system will last for a couple of years at least if you are fine with not maxing out all of your games. For example, I still use the GTX460, which launched in August 2010, and I can still play most games on high settings. Settings such as anti aliasing have to be kept low, or disabled when using FXAA/SMAA, but games are still graphically far superior compared to the consoles. Of course that will change once the first true next-gen games launch, but again, if you choose the hardware wisely, it will last for more than just a few months. A High-End PC that is able to max out every game won't turn into a Low- or Mid-End system in a few months.
-some gamers say good graphichs card is the most important hardware for PC gamer. Not so!
-for Motherboard is the backbone of your PC. To be able to run games fast, you need a high end motherboard that has gotten good rewiews in mags. Do not be stingy here!
Graphics card: YOu can actually go for older model, year 2012 even. If you have a recent motherboard and good Processor, even a decent older graphics card will do, and will be a big saving in your budget.
Memory: Do not overdo it. Too much memory can actually hamper your PC. Just few sticks will do, and they need not be the latest models.
I disagree with some of your points. It doesn't take a High-End motherboard to run games acceptably. A good motherboard for ~$100 or even less is good enough, though it depends on the model and the reviews. I purchased a MSI Z87-G43 for about 90€ in September and it works just fine. While it's true that the processor plays an important role in a PC, it isn't the most important part if you like to play videogames. Graphic-cards are more important in gaming, and 'go for an older model, year 2012 even' is honestly a terrible advice.
What model specifically? It makes a huge difference if you talk about, for example, a GT620 or a GTX670, so define 'decent'. The creator of this topic wants a gaming system, which means the best thing to do is going with a GPU of the latest generation. It doesn't need to be the highest possible model, but a GTX760 or a R9 280(x) would be a good choice for gaming on high to very high settings. That are just examples, though. It depends on the budget.
As for Memory, 8GB of RAM is enough for gaming, there is no need for more. Only 64-bit programs can utilize more than 2GB of system RAM, and there aren't many games that can do that yet. But, like so many things, that will change because of the next-gen consoles.