Davidtco007 :
hi, which RAM size is best for a pentium 4 CPU with processor 3.0 GHz?
It largely depends on the CPU socket and your motherboard. The early P4 3GHz were socket 478 and were limited to a max of PC3200 DDR 2GB on most boards, I think some may take 4GB. The newer P4's were LGA775 and could take upto 8GB, again, depending on the motherboard.
Best you can do is to look in your motherboard manual or google it for a manual, if you don't have it.
Although, if you're running a P4 of any description, you're almost certainly running Windows XP and 2GB should be enough and 4GB is ideal (it won't recognise 4GB, only about 3.5GB).