LogFAQs > #965402226

generally you can mix and match ram so long as they are the same generation (ddr4, ddr5 etc). you can also generally mix and match and run in dual channel. but they will bottleneck to whatever is the slowest. you'll still get the full capacity though (i.e. 4+8 will get you 12, not 8)
thats a very brief overview, it is a little more complex than that but not to a major extent