I have a pre-built h8xt HP desktop intended for gaming. I'm running Windows 7 64-bit. I want to upgrade the current card (AMD Radeon 7570) to a GeForce GTX 670. My question is if this is possible because my computer is a pre-built. I would have to improve the power supply from 300w to 500w, so upgrades include replacing the power supply and graphics card altogether. Do I have to buy these two new components through HP rather than somewhere like Newegg? And if so, does my computer allow for self-modifications and upgrades?