Most of the time, you can't.
Tell me how you can upgrade your processor to 2x the performance for the same number of cores?
You can't. You buy a new CPU, and with that, probably a new machine.
If you want a new CPU, and the board doesnt support the socket, you get a new board, easy. Why re buy every other component, makes no sense at all
Games CPU, is that even a thing? It only runs games? It has special features that shoot lasers?
You can't replace just the GPU and expect better everything, all you are going to get, it's better frame rates, and also the GPU will be bottlenecked because your CPU isn't capable of doing the AI, game logic, etc. and then make all the draw calls of your brand new Graphics Card.
Also, the memory won't be able to fill the graphics card memory in time for the frame.
So all soft are and games require the same balance of CPU and GPU, again rubbish. If your CPU is sound and the GPU could do with a bump, bump it.
Again, why rebuy everything else in the box?
What's the matter of an SSD on a Core 2 Duo, yes, it will make it faster, but it won't make it a new processor, it won't make the whole system better. Yes, it's cheap, but it's better to put that money on the piggy bank and spend on a next generation computer when your PC will be obsolete.
It will boot faster, respond faster, nice upgrade. For games that read data such as FSX, it will make a big difference. FSX is quite specific, but a cheap SSD is a nice upgrade for any PC
Also, gaming on PC, is a waste of time and money.
You get so less bang for buck than a console, you have problems, the games are made to be as heavy on GPU's as possible, for the hype, and for the GPU manufacturere's money, which is evident. Whilst on consoles, they keep going and going year after year, you'll never have the urge to update your hardware to just play a new game... But that's me, I'm 27, so...
"You have problems" So, you decide that games software is always a problem?
Graphic quality and frame rate are better on a gaming rig. Costs more sure but if you are into gaming, you pay for it. Far more support for add ons, far more features within the game. Supports more peripherals. You can upgrade your PC as required as games get better and better, console upgrades are years and years apart
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Past history is showing us the opposite. Apples profits are higher then competitors, even though they have more market share. Apple still has marketshare in the $1K and above category, which again has much higher profit margin.
What is Apples market share in +$1k?