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the problem with this is 15dba= really quiet. a harddrive is ~30dba... 30dba + 15dba = 30.00001 or something stupid. >>
Right. The total noise is to a great extent determined by the loudest source of noise. This means you have to quiet down everything, including HD, case fans, PSU, not just CPU fan. Sometimes your video card fan can be the loudest, as was the case with my GF2Ultra, which I got rid of.
MowSow, in your setup, I think you'd better replace 75GXP with Seagate Barracuda IV and/or use a drive enclosure like
SilentDrive. If you really need RAID, then you might want to avoid Barracuda IV, though.
Your PSU, which happens to be the same one as I have, may not be the quietest one on the market, either. You might like to replace it by Enermax modded with Papst fan sold by noisecontrol.de or quietpc's PSU or the FCA version of Enermax.
As far as CPU goes, since Tualatin Celeron runs really cool even if overclocked, much cooler than Athlon XP, it may be better to go with it. You might be able to run without a fan if you are lucky. Even if you aren't, you can run it with a stock Intel HSF, which is said to be quiet, or use your Alpha6035 with Papst 8412 NGL using a 60mm-to-80mm fan adapter (you may have to replace your clip for Alpha too). Performance of Tualatin Celeron isn't so bad really, particularly if you overclock it.
In general, the less heat, the less cooling required, hence the less noise.
BTW, I'm not an Intel fan at all and currently running a Thunderbird 800Mhz@901Mhz with Abit KT7. I'll probably upgrade this box with Athlon XP or P4 Northwood, but I'll use a Tualatin Celeron or PIII with my second box for the maximum quietness.