System start fail

Hi guys, currently having a issue where my computer will refuse to boot without reseating the RAM - usually but not every time prior to this happening after shutting down my machine I will still have my desktop lights on and hardware in use (I can hear and see this) and the power button will...

I'm currently building my PC, I already have all but the hardest part, which is GPU. I want it to be the right card, as I am honestly overwhelmed by the amount of GPUs available on the market, and I also don't want to buy something i wil regret in a few months. My budget is around 200$. I will...