I strongly advice you not to buy most OEM systems. Specially Dell and HP. I will explain why.

Custom components

Very often big stores like Dell or HP will build and put custom components in their PC. As result it is not possible to upgrade them. Imagine you bought Dell PC. After some time, you would like to upgrade your video card. But this video card requires more power, so you have to upgrade power supply. But they use custom power supply with different mounting, so it is simply not possible to unscrew old one and put new one. So, you will have to buy power supply from Dell and that will be much more expensive. And actually, in some

Imagine you are tough on money but would like to buy computer. And you see relatively cheap OEM system with say Intel Core i5-4690, 8GB DDR3-1600 memory. You think that i5 has specific specs and should work with same speed everywhere. But after you bought it you will be unpleasantly surprised.

Check out this video: https://www.youtube.com/watch?v=6ESkYHzT8LU

As you can see in this video, after they moved CPU and memory to different motherboard and power supply, performance of system increases a lot. Basically, Intel allow to fine tune power package and while it will be same Core i5-4690 it will work much slower in such OEM system with tuned down power package.

Firstly, I blame Intel for this situation. The