building a reliable system?

hi there... bear with me. i have been building systems for about 8 years, but haven't done many in the past couple years. i am long overdue for an upgrade, but i have some questions.

i value reliability quite a bit. the current system i have is an OLD amd athlon 2400 and it has worked flawlessly for five years plus! crazy, i know.

anyway, the last two systems i built were based on the Pentium D 940 Presler, with Abit Intel 945p boards, and hard drives in a raid mirror array. they were both for family members and they have both been headaches. i'm not exactly sure where to pinpoint the problems, but there have been overheating issues (poor hsf? psu?), raid failures (drives? mobo?), sound goes out, etc... so i guess i'm asking, what did i do wrong?

and as i look forward to building my own replacement, what should i look for to make a reliable system? i'm looking at the intel quad cores with the 45nm and the 95w (9000 series i think).

feedback is much appreciated.

thanks for the informative read. i'll be taking several of your suggestions into consideration. i guess my main concern was not repeating some of the mistakes from some of the builds from a year ago. best guess on what was the source of my woes with those builds? thanks.

yeah... psu should be good. it was the standard antec truepower or smartpower or something that comes with the case. i'm thinking it was the mobo. looks as if abit is no longer in business... and i'm guessing because of the wide range of problems that i was having that they were all linked to the board. ok... thanks for taking the time.