I want to buy new computer, but I have low budget. I have to consider about used computer HP Compaq Elite 8300 SFF with Intel Core i5 3470 or assemble my own with AMD Athlon x4. Which processor is better? And if I want to use Radeon RX 460, which processor will be better to work with? Also, I have read about locked BIOSes in HP computers that not allow to add new hardware. Will new graphics be usable in it? Has the motherboard some unlocked BIOSes that allow new hardware?
Pre-built systems from major manufacturers like HP have a BIOS with limited user options. Overclocking is pretty much out as far as options go. As long as the BIOS has the microcode for a CPU (assuming the same socket and other system requirements) it will work. New parts, again assuming the system as a whole isn't a limitation, should be fine. Depending on the video card, a new power supply may be warranted.