I know my question covers more than just mobo and memory but I think these 2 components are the most crucial. The software is also an issue. I want to build a new system that is optimized for opening a lot of applications and files at the same time. I often have as many as 50 webpages open at one time, am watching some video files to do editing, have tbird open for emails, wordpage, skype, msn, qq, facebook, a couple of word docs, video processing (editing or rendering) I have an amd dual core II cpu with a gigabyte all in one mobo and 4gb of ram. Last year I upgraded from a Pentium D and did not notice any significant improvements. I originally had XP pro on this sytem but installed Linux Mint to see if I could realize any improvements. It was noticeably faster booting up but did not seem to be any faster or slower when I was up and running.
I know that separately, each of the tasks that I do work well on a P4. I know because my old faithful notebook is a P4. What I need is a pc that is very stable and quick when all of these things are open at the same time.