definitely the more i look at it, the nicer underclocked athlon64 gets. affordable, lots of stock parts/chipset support... scientific linux (https://www.scientificlinux.org/) seems a decent bet to run on it (or maybe ubuntu...)
@oralpain: many thanks for all these suggestions. anyone recommend any other possible a64 motherboards that allow simple underclocking?
slowly speccing out a few of the smaller bits and pieces now i'm getting the flavour...
this looks good for passive cooling: Scythe Ninja Fanless Heatpipe CPU Cooler http://www.quietpc.com/uk/amdcooling.php