Socket 939 would be a better bet. 940 limits you to opeterons and some of the FX chips, plus all the socket 940 motherboards will need expensive Registered memory (as they're basically Server boards)
You simply get a bit more flexibility with your choices if you go for 939 (or even 754)
Opteron and <i>clawhammer</i> core A64 are basically the same chip. 'Newcastle' has less cache, but it's not that big an issue with A64 chips because the on-die memory controller makes memory access much faster.
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Epox 8RDA+ V1.1 w/ Custom NB HS
XP1700+ @200x10 (~2Ghz), 1.4 Vcore
2x256Mb Corsair PC3200LL/1x512Mb Corsair XMS PC4000 2.5-3-3-7
Sapphire 9800Pro (VGA Silencer Rev3) @418/742