Cooling. Lots of it. Athlons run notoriously hot. I can't recall the temp of my T900, but ISTR that it was around 50-55C. I'm running a golden orb on it - which whines a bit - I sometimes wonder if people realise that their fans are run with a horizontal axis.
Drive coolers for the disks are also a must - particularly if you're using 7200rpm or 10Krpm (or even 15Krpm) disks. Some CD-RW drives (in particular Yamaha ones) run very hot - make sure there's enough airflow there. Put an extra case fan in if possible.
Then, make sure you've got a really good PSU. If you get them with the case, they sometimes skimp a little. I've got one from Enermax, and other people have also said they're good - a little pricey, but good.
Roger - not necessarily speaking for empeg
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roger